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
Merge "Verify that CONF.compute_driver is defined"