Revert "Revert "change the default to PyMYSQL""
Revert "Revert "change the default to PyMYSQL""

Some projects (Neutron) seem to be affected more than others, so we should revert this
to allow for a more selective choice of the DB driver on a per project basis.

We can re-enable the use MySQL-python just for Neutron.

This reverts commit de8d29ed8ce4a26b61cbee48f9fe5418d5416a06.

Related-Bug: #1464612

Change-Id: I889f4f8b116c413b300ab9eecc7b428a9a4afb1a

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