Specify zone when adding to an instance-group
Specify zone when adding to an instance-group

As the zone might differ from the default zone (in fact, it does for
staging), gcloud cannot guess the zone otherwise.

Change-Id: Iae9525340d77f23be60a4ccf7ac6cfaff5b526ca

bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration