Sync OPENSTACK_KEYSTONE_DEFAULT_ROLE with keystone

For now, keystone default role is _member_, while horizon set
OPENSTACK_KEYSTONE_DEFAULT_ROLE to Member. It will really be user
friendly to modify horizon default value to _member_ to sync with
keystone's default setting.

Change-Id: I55d15e6cfb74e52e933c5a44efd6c27930415738
Closes-Bug: #1264228
Merge "Sync OPENSTACK_KEYSTONE_DEFAULT_ROLE with keystone"