gke-disk-image-builder: Configurable service account

[nstogner]

Customer need: need to be able to use a preexisting GCP Service Account when running the image builder instance.

For example, what I did:

                                                Instance: compute.Instance{
                                                        Name:        fmt.Sprintf("%s-instance", name),
                                                        MachineType: fmt.Sprintf("zones/%s/machineTypes/%s", req.Zone, req.MachineType
),
+                                                       ServiceAccounts: []*compute.ServiceAccount{
+                                                               &compute.ServiceAccount{
+                                                                       Email: req.ServiceAccount,
+                                                                       Scopes: []string{
+                                                                               "https://www.googleapis.com/auth/devstorage.read_only"
,
+                                                                               "https://www.googleapis.com/auth/logging.write",
+                                                                               "https://www.googleapis.com/auth/monitoring.write",
+                                                                               "https://www.googleapis.com/auth/pubsub",
+                                                                               "https://www.googleapis.com/auth/service.management.re
adonly",
+                                                                               "https://www.googleapis.com/auth/servicecontrol",
+                                                                               "https://www.googleapis.com/auth/trace.append",
+                                                                       },
+                                                               },
+                                                       },