Allow databases to be created outside upgrade_schema scrpt.

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