Fix migration 185 to work with old fkey names

Migration 133_folsom.py accidentally changed the name of the FK
constraint on instance_info_caches 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 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
Merge "Fix migration 185 to work with old fkey names"