Downgrade MySQL to the same state it used to be

virtual_interfaces_instance_uuid_fkey exists as a KEY, not a UNIQUE KEY in
MySQL which causes trouble when downgrading.

Don't re-create the virtual_interfaces_instance_uuid_fkey on downgrade as it
wasn't upgraded properly and therefore still exists. This forces the downgrade
to look exactly like it was before the upgrade.

Fixes bug 1207344

Change-Id: Id1700e1cbba669df2dd8e5c5a842b859577aa32c
Merge "Downgrade MySQL to the same state it used to be"