Sync default OPENSTACK_KEYSTONE_DEFAULT_ROLE with keystone

bug 1264228 changes OPENSTACK_KEYSTONE_DEFAULT_ROLE in example
local_settings.py and the document from "Member" to "_member_"
which is the default config value in keystone.
However, OPENSTACK_KEYSTONE_DEFAULT_ROLE in settings.py is not
changed and the default value is still "Member".

Change-Id: I4bd4e2388bb2678f80e4f744682699f69252a732
Closes-Bug: #1296429
Merge "Sync default OPENSTACK_KEYSTONE_DEFAULT_ROLE with keystone"