Move switch roles in teardown to base rbac classes

There is no need to add switch_role(switch_to_rbac_role=False) in the tearDown method of every single Patrole test class. It is more sensible and more maintainable to move switch_role(switch_to_rbac_role=False) to every base Patrole test class, e.g. to rbac_base.py in compute, identity, image, network and volume project folders.

The only edge case can be illustrated by the following example:

    def tearDown(self):
        rbac_utils.switch_role(self, switchToRbacRole=False)

        # Guarantee that the server is active during each test run.
        vm_state = self.client.show_server(self.server['id'])['server'][
            'OS-EXT-STS:vm_state'].upper()
        if vm_state != 'ACTIVE':
            self.client.resume_server(self.server['id'])
            waiters.wait_for_server_status(self.client, self.server['id'],
                                           'ACTIVE')

In this case, the call to rbac_utils.switch_role() should be removed; instead, super(...).tearDown() will be called before any test-specific clean up.