[Context] Add region_name in credential
[Context] Add region_name in credential

If we don't add region_name in multi-regions cloud, it exist the
potential possibility that we can not pass the validate function.
And previous param of region_name does not in right location.

Change-Id: Iaa5d3a3efab9ca83ab5d78f119df0ee912b9aa7c

yeming233/rally,yeming233/rally,openstack/rally,openstack/rally,openstack/rally,openstack/rally