Summary:
Current scripts make it hard to administer Phabricator instance while not having
direct (priviledged) access to the database. This change allows scenario where
DB administrator creates the databases for you before you run update_schema
script.
Test Plan:
Create the databases before running the update_schema script - it shouldn't
complain that the databases already exist.
Reviewers: aran, epriestley
Reviewed By: epriestley
CC: aran, epriestley
Differential Revision: 1014
