When nova-api start and the nova database hasn't yet been created, it
would create it itself, preventing the migration in nova-conductor to
run and making it unable to start. This commit ensures that nova-api
container will not try to steal nova-conductor's job of creating the
nova database.

Nova-scheduler reacts differently to the missing database and simply
exits with a backtrace. This commit also prevents nova-scheduler to
start when the nova database cannot be found.

Change-Id: Ib6f7521151815faa7a26ec1d170df5a1cdb7fd63
Related-Bug: #1335804