one-click failover for app server

Should do the following
- Playbook on proxy to point to replica
- Update primary and replica server docs (primary checkbox)
- Update server field for all sites on the server
- Update bench field for all sites -> should consequentially update bench config on server too
- Bring up all the containers on replica (better if images are already syncing during replication)
- Remove primary's key from replica
- Update release groups to replace primary with replica
- Regenerate nginx conf on replica