Code review comment for ~pguimaraes/maas:db-fail-lp-1843493

Hey blake-rouse, thanks for the review.
Well, I am essentially seeing two kinds of errors: PodProblem-related (which is expected, nothing wrong here); and OperationalError (check: https://bugs.launchpad.net/maas/+bug/1843493/comments/2)
The later is a database issue, actually, which I think comes before libvirt setup, since libvirt is configured once I fire an RPC to provisioningserver, right?

I don't think I am near the solution however, I'm still seeing DB conflicts, even if I set my thread to sleep for several seconds (which is recommended approach as per postgres docs).

I feel we're missing a "with transaction.atomic():" somewhere to make all other pod allocations to hold. However, whenever I add this to compose method, it breaks with TransactionManagementError.