Allow _check_instance_build_time to hit db slave

If your deployment has a slave configured, _check_instance_build_time
will read against the db slave. If you have no slave configured
then behavior remains unchanged.

Change-Id: I2e19bef2b4231a711f473807a14a13398cbc7278
Partially-implements: blueprint periodic-tasks-to-db-slave
Merge "Allow _check_instance_build_time to hit db slave"