Add MySQL-python dependency
    
    This is to allow other packages that already depend on python-oslo-db to
    get rid of that dependency.
    
    Note that technically speaking oslo.db does not require the package too,
    because we have plenty of options to deploy OpenStack, using other
    database backends different from MySQL, or just using other Python
    drivers for SQLAlchemy.
    
    Ideally, this added dependency should be dropped after a while, when all
    installers install the proper Python driver for the type of setup
    they deploy.
    
    Change-Id: I1d1852c993e6e1a28fbe9e0f693864471213c18e