Add os_region_name configuration option for multiple regions support

For multiple regions support, you need to supply the os_region_name in
nova.conf refer community issue
https://bugs.launchpad.net/nova/+bug/1087735.
Add this option to the nova.conf template and set it with the node
["openstack"]["region"] attribute.

Closes-Bug: #1316452
Change-Id: I8afdd4f621ceecff8da25e285b9fc07d258dadef