Test oslo.db against all supported SA branches

Currently we test only the latest supported SA branch in the gate,
which means regressions for previous versions might be introduced.

Depends on I4128272ce15b9e576d7b97b1adab4d5027108c7c to be merged
to oslo.db first.

Change-Id: Ia09e0c13db20ab6974d5c72d3f13363193c56f4d
Merge "Test oslo.db against all supported SA branches"