Set up 'compute_task' conductor namespace.

This patch sets up a new API to be exposed by the nova-conductor
service.  The existing methods exposed by conductor are all proxy
operations on behalf of compute nodes.

For Havana, we have plans to add methods that go the other direction.
Operations initiated by the API will be processed by the conductor,
which will include telling one or more compute nodes what to do.  By
putting all operations of this form in their own API, it will keep
things a bit more clearly separated.

Change-Id: Ifa3d443b9bd7f406a7a3bd837351bc689bf2b6c8