Structure only

This MR follows a conversation with @jarv around the structure.

It:

1. Defines the structure for the scripts a little better
2. Adds pointer to the script folders in the issue (follow the 📁icon to get to the folder)
3. Moves all scripts over to only run on GCP. Any script which needs to access Azure will need to do so using knife ssh
4. Makes all the script references full qualified paths under /opt/gitlab-migration. This directory is maintained on the deploy nodes by Chef.
5. Adds instructions for how the failover 🐺 Coordinator can start a new failover attempt.

cc @jarv @nick.thomas @digitalmoksha