Add `post_downgrade` hook for migration tests.

This can be used by tests to assert the state of the database after a
`downgrade` has been run.

This patch also renames `_prerun_#` to `_pre_upgrade` to match.

Change-Id: Id2a100a8270835d1ef6f9908ff5b89e2b531bb3c
Migration 146: Execute delete call.

The original code constructed but did not execute the call to delete the
availability zone metadata records.

The fix is to call `execute`.

Change-Id: I3a5e2e7fd6e96b66b2073a35b55eec2002c6ae58
Merge "Add `post_downgrade` hook for migration tests."
Merge "Migration 146: Execute delete call."
Merge remote-tracking branch 'm_src_repo/master' into build-temp