We should only setup the cellsv2 environment if configured to do
so (which will mean the DB was actually setup, which is a requirement).
The related nova change with the release note for this is:
a86d78f5f048e11ddb7e375f2d6cc707108b9478
Change-Id: I090d5994c3077d86c60457e4c16a03d35e0c409c