Env vars aren't validated

[jacobweinstock]

Env vars are not validated like cli flags are.

Expected Behaviour

Env vars should be validated just like cli flags are.

Current Behaviour

A bad env var that wouldn't pass the cli flag validation could cause unexpected behavior.

For example:

# cli flag validation
❯ go run main.go server --bmcTimeout 60 
Error: invalid argument "60" for "--bmcTimeout" flag: time: missing unit in duration "60"
Usage:
  pbnj server [flags]

Flags:
      --bmcTimeout duration        Timeout for BMC calls (default 15s)
      --enableAuthz                enable Authz middleware. Configure with configuration file details
      --enableHTTP                 enable the HTTP server
  -h, --help                       help for server
      --hsKey string               HS key
      --metricsListenAddr string   metrics server listen address (default ":8080")
      --port string                grpc server port (default "50051")
      --rsPubKey string            RS public key

Global Flags:
      --config string     config file (default is pbnj.yaml)
      --logLevel string   log level (default is info (default "info")

invalid argument "60" for "--bmcTimeout" flag: time: missing unit in duration "60"
exit status 1

# no validation for env var. The server starts up with the bmc timeout set to 0
PBNJ_BMCTIMEOUT=60 go run main.go server
{"level":"info","ts":1624559879.264104,"caller":"cmd/server.go:97","msg":"debugging","service":"github.com/tinkerbell/pbnj","timeout":0,"timeout_string":"0s"}
{"level":"info","ts":1624559879.2745972,"caller":"grpcsvr/server.go:129","msg":"starting PBnJ gRPC server","service":"github.com/tinkerbell/pbnj"}

Possible Solution

Steps to Reproduce (for bugs)

Context

Your Environment

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