Load settings from nova-compute.conf

As of the Grizzly release, UCA nova places the compute_driver
setting in nova-compute.conf. Without this setting, cobalt-compute
fails to operate.

This patch adds a flag to the invocation of cobalt-compute so
that settings are loaded from nova-compute.conf in addition
to nova.conf.

Change-Id: I1fe1e95820f76dbc48798dc116176dd61441c489