Fix migration 185 to work with old fkey names

Due to different versions of MySQL it is possible to end up with
different FK's then what is expected. For example the FK constraint
on instance_info_caches changed from instance_info_caches_ibfk_1
to instance_info_caches_instance_uuid_fkey and on virtual_interfaces
from virtual_interfaces_ibfk_1 to
virtual_interfaces_instance_uuid_fkey meaning databases who have
upgraded from before folsom may have the old fkey.

This patch adds a check for those with the old fkey name to make sure
it is dropped before changing unique constraints (as is required by
MySQL). It also fixes the fkey name to be as defined in 133 if it
wasn't before (which will persist on downgrade being consistent with
those who have come in after folsom).

Closes-Bug: #1245502

Change-Id: Ib0dcd04fcb9ed776c76d40561181abad2e14f76f
(cherry picked from commit c620cafb700ca195db0bd0ef9d62a0c9459bdc38)
Merge "Fix migration 185 to work with old fkey names" into stable/havana