nova: use the correct notification driver

Nova now uses oslo.messaging and not the Oslo RPC code anymore,
therefore the new driver should be used instead.

Change-Id: I3533975ad38ff99bee6cfaa5332843444650f61f
Merge "nova: use the correct notification driver"
Merge commit 'refs/changes/71/70571/2' of git://gold.eng.hq.xensource.com/git/github/openstack-dev/devstack into tmpbranch
Merge commit 'refs/changes/99/70999/4' of git://gold.eng.hq.xensource.com/git/github/openstack-dev/devstack into tmpbranch