Move mysql_engine option to [database] group

We can not move the remaining db-related options to the [database] group
because they are defined in oslo-incubator.

sqlalchemy/models.py incorrectly sets options as default.
That's why we need to move mysql_engine to [database] group

Fixes: bug #1193499

Change-Id: Ic485fb28279e5ceeed6e21b4dee2a37a9eea6d6c