Verify that CONF.compute_driver is defined

The compute_driver parameter is a mandatory parameter only for nodes
running the Nova compute service. However Other Nova services like
Nova API actually call the compute_driver_matches() function.

Change-Id: Iac7cd46c6bdb4268cfbc650dcde0c95e69545f04
Fixes: bug #1184641
(cherry picked from commit fa5079c257c00b8b56138233aca5755c54db9143)
Merge "Verify that CONF.compute_driver is defined" into stable/grizzly