Change default region for placement to empty string

As per tempest design, if tempest conf is not having any region for
a service then identity region should be used. In case of placement
the default value is "RegionOne" which is considered as placement
region if region missing in tempest conf. In order to have identity
region to be used as default we need to change to empty string for
placement service. Empty string can be seen being used in other
services like volume, image etc.

Please check the below link for the logic of default region
https://github.com/openstack/tempest/blob/master/tempest/config.py#L1435

Change-Id: I2a1045cd0f9eb830493b2b4287ccee46223f3c59

releasenotes/notes/change-default-region-for-placement-to-empty-string-394f1132c28345bb.yaml

@@ -0,0 +1,13 @@
+---
+upgrade:
+  - |
+     Default value of config option ``CONF.placement.region`` is updated
+     from ``RegionOne`` to empty string.
+
+     As per tempest design, if tempest conf is not having any region for
+     a service then identity region should be used. In case of placement
+     the default value is "RegionOne" which is considered as placement
+     region if region missing in tempest conf. In order to have identity
+     region to be used as default we need to change to empty string for
+     placement service. Empty string can be seen being used in other
+     services like volume, image etc.

tempest/config.py

@@ -390,7 +390,7 @@ PlacementGroup = [
                default='placement',
                help="Catalog type of the Placement service."),
     cfg.StrOpt('region',
-               default='RegionOne',
+               default='',
                help="The placement region name to use. If empty, the value "
                     "of [identity]/region is used instead. If no such region "
                     "is found in the service catalog, the first region found "