Lay cluster.databases configuration groundwork for database clustering

Summary:
Ref T4571. This adds a new option which allows you to upgrade your one-host configuration to a multi-host configuration by configuring it.

Doing this currently does nothing. I wrote a lot of words about what it is supposed to do in the future, though.

Test Plan:

• Tried to configure the option in all the possible bad ways, got errors.
• Read documentation.

Reviewers: chad

Reviewed By: chad

Subscribers: eadler

Maniphest Tasks: T4571

Differential Revision: https://secure.phabricator.com/D15663