Wait for another deploy to finish

[chilledornaments]

It would be convenient if the plugin checked for an existing on-going deployment and optionally waited for it to finish before proceeding.

Maybe we could add a setting for in_progress_build_behavior with an option to either "wait" or "cancel", with "wait" being the default. When the plugin detects another in-progress build and in_progress_build_behavior is set to "wait", the plugin would simply poll the deployment status on a long interval (60 seconds sounds reasonable). We would probably also need an in_progress_build_wait_timeout to end polling after a period of time.

When in_progress_build_behavior is set to "cancel", the plugin would simply log that there's another ongoing build and os.Exit(1)