Bugfix missing foreign key removal for mysql

Downgrading the cinder schema fails when running 018_add_qos_specs.py
under MySQL. The upgrade path of this schema patch adds the foreign
key volume_types_ibfk_1 on table volume_types, and the downgrade does
not correspondingly remove it before attempting to drop the
qos_specs_id column.  This update removes the foreign key when
the engine is mysql prior to dropping the column.

Change-Id: Ibd3b35ad3b0bd41ad04ab7aeeb28c3ba7e5d255d
Closes-Bug: #1264360