Document and/or add warning for sidecar configs with the same name

[kasia-kujawa]

Document and/or add warning for sidecar configs with the same name, related to #51 (comment)
Currently when 2 sidecar configs have the same name then the second config overwrites the first.

E.g.
TailingSidecar

apiVersion: tailing-sidecar.sumologic.com/v1
kind: TailingSidecar
metadata:
  name: tailingsidecar-sample
spec:
  configs:
    sidecarconfig:
      volume: varlogconfig
      file: /varconfig/log/example1.log
    sidecarconfig:
      volume: varlogconfig
      file: /varconfig/log/example2.log

Pod:

apiVersion: v1
kind: Pod
metadata:
  name: pod-with-annotations
  namespace: tailing-sidecar-system
  annotations:
    tailing-sidecar: sidecarconfig
  labels:
    app: pod-with-annotations
spec:
  containers:
  - name: count
    image: busybox
    args:
    - /bin/sh
    - -c
    - >
      i=0;
      while true;
      do
        echo "example0: $i $(date)" >> /var/log/example0.log;
        echo "example1: $i $(date)" >> /var/log/example1.log;
        echo "example2: $i $(date)" >> /varconfig/log/example2.log;
        i=$((i+1));
        sleep 1;
      done
    volumeMounts:
    - name: varlog
      mountPath: /var/log
    - name: varlogconfig
      mountPath: /varconfig/log
  volumes:
  - name: varlog
    emptyDir: {}
  - name: varlogconfig
    emptyDir: {}

Created Pod by operator:

$ kubectl describe pod pod-with-annotations -n tailing-sidecar-system 
Name:         pod-with-annotations
Namespace:    tailing-sidecar-system
Priority:     0
Node:         sumologic-tailing-sidecar/10.0.2.15
Start Time:   Mon, 15 Mar 2021 14:32:31 +0000
Labels:       app=pod-with-annotations
Annotations:  cni.projectcalico.org/podIP: 10.1.37.83/32
              cni.projectcalico.org/podIPs: 10.1.37.83/32
              tailing-sidecar: sidecarconfig
Status:       Running
IP:           10.1.37.83
IPs:
  IP:  10.1.37.83
Containers:
  count:
    Container ID:  containerd://1b56d9f4198a8f0ab365a2cda8454598301e1422b9a8c102edbc14bba6ba512e
    Image:         busybox
    Image ID:      docker.io/library/busybox@sha256:ce2360d5189a033012fbad1635e037be86f23b65cfd676b436d0931af390a2ac
    Port:          <none>
    Host Port:     <none>
    Args:
      /bin/sh
      -c
      i=0; while true; do
        echo "example0: $i $(date)" >> /var/log/example0.log;
        echo "example1: $i $(date)" >> /var/log/example1.log;
        echo "example2: $i $(date)" >> /varconfig/log/example2.log;
        i=$((i+1));
        sleep 1;
      done
      
    State:          Running
      Started:      Mon, 15 Mar 2021 14:32:33 +0000
    Ready:          True
    Restart Count:  0
    Environment:    <none>
    Mounts:
      /var/log from varlog (rw)
      /var/run/secrets/kubernetes.io/serviceaccount from default-token-smcv9 (ro)
      /varconfig/log from varlogconfig (rw)
  tailing-sidecar0:
    Container ID:   containerd://6ab75471db047be85bf86353a292ca51a2093fe654f1e6dd92aa6bcce7acad3f
    Image:          localhost:32000/sumologic/tailing-sidecar:demo
    Image ID:       localhost:32000/sumologic/tailing-sidecar@sha256:0ed3d789b7e9a23a61cba26d394af02505fc2d25a96fdd85f03cd4bf2b3ca53a
    Port:           <none>
    Host Port:      <none>
    State:          Running
      Started:      Mon, 15 Mar 2021 14:32:34 +0000
    Ready:          True
    Restart Count:  0
    Environment:
      PATH_TO_TAIL:     /varconfig/log/example2.log
      TAILING_SIDECAR:  true
    Mounts:
      /tailing-sidecar/var from volume-sidecar0 (rw)
      /var/run/secrets/kubernetes.io/serviceaccount from default-token-smcv9 (ro)
      /varconfig/log from varlogconfig (rw)
Conditions:
  Type              Status
  Initialized       True 
  Ready             True 
  ContainersReady   True 
  PodScheduled      True 
Volumes:
  varlog:
    Type:       EmptyDir (a temporary directory that shares a pod's lifetime)
    Medium:     
    SizeLimit:  <unset>
  varlogconfig:
    Type:       EmptyDir (a temporary directory that shares a pod's lifetime)
    Medium:     
    SizeLimit:  <unset>
  default-token-smcv9:
    Type:        Secret (a volume populated by a Secret)
    SecretName:  default-token-smcv9
    Optional:    false
  volume-sidecar0:
    Type:          HostPath (bare host directory volume)
    Path:          /var/log/tailing-sidecar-fluentbit/tailing-sidecar-system/pod-with-annotations/tailing-sidecar0
    HostPathType:  DirectoryOrCreate
QoS Class:         BestEffort
Node-Selectors:    <none>
Tolerations:       node.kubernetes.io/not-ready:NoExecute op=Exists for 300s
                   node.kubernetes.io/unreachable:NoExecute op=Exists for 300s
Events:
  Type    Reason     Age    From               Message
  ----    ------     ----   ----               -------
  Normal  Scheduled  7m15s  default-scheduler  Successfully assigned tailing-sidecar-system/pod-with-annotations to sumologic-tailing-sidecar
  Normal  Pulling    7m14s  kubelet            Pulling image "busybox"
  Normal  Pulled     7m13s  kubelet            Successfully pulled image "busybox" in 1.373490106s
  Normal  Created    7m13s  kubelet            Created container count
  Normal  Started    7m13s  kubelet            Started container count
  Normal  Pulled     7m13s  kubelet            Container image "localhost:32000/sumologic/tailing-sidecar:demo" already present on machine
  Normal  Created    7m13s  kubelet            Created container tailing-sidecar0
  Normal  Started    7m12s  kubelet            Started container tailing-sidecar0