Enable support for MySQL with test migrations.

This commit makes the necessary changes to enable the migrations
tests to pass with both mysql and sqlite. However the proper mysql
database still needs to be created beforehand for the tests to run
with mysql.

The sqlalchemy alter() method breaks on sqlite when mysql is enabled
for other databases. So for sqlite testing the alter() calls are now
done manually.

This also adds the MySQL-python module to the test-requires list
so that it will get installed for CI and the virtualenv used for
unit tests.

Change-Id: Ic72476b8e6a910c25747641ef1882639019b433e
Merge "Enable support for MySQL with test migrations."