R1:6a4a9bb2d284

When `cluster.databases` is configured, read the master connection from it

Summary:
Ref T4571. Ref T10759. Ref T10758. This isn't complete, but gets most of the job done:

- When `cluster.databases` is set up, most things ignore `mysql.host` now.
- You can `bin/storage upgrade` and stuff works.
- You can browse around in the web UI and stuff works.

There's still a lot of weird tricky stuff to navigate, and this has real no advantages over configuring a single server yet (no automatic failover, etc).