Performance: improve get_role
Performance: improve get_role

Now Keystone visits DB double times (if use cache, Keystone will
firstly visit DB and then visit cache) when the get_role API is
called.

This patch deleted the redundant second call to impove the
performance a little.

Change-Id: Ib863a1e4d402fa486fc2c50d27ec041be3ebf402

mahak/keystone,openstack/keystone,openstack/keystone,openstack/keystone,mahak/keystone,mahak/keystone