Don't set nova.conf values to there default value

No need to set osapi_compute_extension and volume_api_class to there
default values.

Change-Id: I1f5345fa2e0ac3a3fbb96a63f36207d2233d7837
Merge "Don't set nova.conf values to there default value"
Merge branch 'master' of github.com:openstack-dev/devstack