Pickup the region name passed in from heatclient

For standalone heat operation, middleware will need to set the region
from the X-Region-Name header. Then the keystoneclient will use this
region_name instead of the heat_region_name config option.

Closes-Bug: 1223068
Change-Id: Ia859e67cf8c6e0d1ed9d9e7f5eab2d138c6421ef
Merge "Pickup the region name passed in from heatclient"