Squash all migrations into one

Migration #12 have fixed bug
https://bugs.launchpad.net/manila/+bug/1272303,
but introduced a new one. Because that bug
was rather architectural, we need to address
that architectural flaw instead of fixing
bugs as they come.
Closes-Bug: #1275044

Change-Id: I260a90abd1107c62b687118d5d4a01d7199ea56f