Provide a means to add a startup-script for GCE instances
Provide a means to add a startup-script for GCE instances

Added a startup-script option to the launch-gce-image command
to provide an ability to specify a startup script during instance
launch. This can be used for testing cloud-init and similar
testing (see YETI-822).

Jira: YETI-822

brkt/brkt-cli,brkt/brkt-cli