Add a howto listing the procedure to do a postgres switchover

Currently this documents how to do the switchover in staging based on a single test.

Observations and open questions....

1. Installing omnibus in production is causing chef to run gitlab-ctl reconfigure despite the override

• This is still a mystery. But for the case documented here where we're doing rolling restarts and installing while postgres is shut down it's not actually a problem
• It may be a problem in the future once we change the prefix so we can avoid doing these rolling restarts (and just do pg_ctl restart)

1. Shutting down the primary doesn't cause a failover until the 60s expires. What command(s) can we run to trigger consul/repmgrd to do the failover immediately?
2. Are all the role snippets that need to be updated afterwards really necessary?
3. The first is for the rails-db console and we definitely want it to point to a replica.
4. The second is for the deploy node and is currently necessary but should be straightforward to make autodiscovered
5. I do not understand why the third bit is here. Isn't this the info that comes from consul?
6. Having switched once switching back did not work. I had to run gitlab-ctl repmgr standby promote to force postgres01 to become primary again
7. Probably I should also run some commands to check the current consul state and check that pgbouncer is actually configured correctly.
8. PostgreSQL_ExporterErrors alerts in prometheus after the failover?

• Fixed gitlab-cookbooks/gitlab-exporters!9 (merged)

1. The slot seems to be leftover on the old primary after setting it to follow :(