Support PhabricatorOpaqueEnvelope for managing database passwords

Summary: Currently, MySQL/MySQLi connections store passwords in plain text on the object. Allow them to be stored in PhutilOpaqueEnvelopes instead. See D3053.

Test Plan: Loaded site.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D3054