In multi-region clouds, Neutron needs to understand which Nova
region it should send notification events to, otherwise its not
able to consistently resolve the correct Nova API endpoint to
use to send port creation notifications to.
Add region_name to nova section based on configuration of charm.
Closes-Bug: 1594602
Change-Id: If4d6ba41a064bc93eaa32efcd524fea6f0891275
(cherry picked from commit 9e12acb3b8)