Fully supporting Remote Instances and clean-up of Options

Script now fully supports Remote Instances with [-r <remote_instances>] option - no need to pass in Instance name and zone.

Removed [-t <label_name>] as this functionality can be achieved using [-f <gcloud_filter_expression>] (which was previously [-T <gcloud_filter_expression>].

Also added Dry run for debugging.

Large amount of code was changed.