Add a "ClusterDatabase" Almanac service type

Summary: Ref T5833. This doesn't do anything yet, but will allow new instances to automaticaly bind to an open database without anything too hacky.

Test Plan:
Created a service of this type.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5833

Differential Revision: https://secure.phabricator.com/D11372