Make "mysql" mean "Ferret engine" in Fulltext search

Summary: Ref T12819. Swaps constants so existing configurations that use a "mysql" engine now use the Ferret engine, not an InnoDB/MyISAM FULLTEXT engine.

Test Plan: Swapped my local config back to "mysql" (the default), saw Ferret engine results in the UI.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12819

Differential Revision: https://secure.phabricator.com/D18590