PyMySQL is prefered to PythonMySQL for Sqlalchemy, as it provides
python3 support and is actively maintained, and is therefore the
currently recommended lib for db connections.
* https://wiki.openstack.org/wiki/PyMySQL_evaluation
Kolla currently uses PyMySQL for all connections bar Barbican
(which works fine with PyMySQL): once this commit is merged it will
be possible to remove the PythonMySQL libs, and mysql libs for kolla
images (except kolla-toolbox).
TrivialFix
Change-Id: Id256387134ca551a181c5e49c9b6d63f62b72523