Remove dependency on MySQL-python.

Tempest does not depend on MySQL, it depends on sqlalchemy. If the
cloud it's on was installed using MySQL, then MySQL will be instaled.
If it was installed using PostGres, then postgres will be instaled.
This dependency is misleading.

Change-Id: I9ad5fd3268fd35e20ec87a800123e522e34844c4
Merge "Remove dependency on MySQL-python."