Passthrough-enable nova.conf.

This permits passthrough settings to be applied to nova.conf using
'nova' as the key in the config. I'll do a bulk patch for this once
reviewers have had time to digest and talk about this.

blueprint: passthrough-config
Change-Id: I3bb935f16e8d742aa8acc50c52accbd033488f02
Merge "Passthrough-enable nova.conf."