Add RBAC tests for extend volume policy actions

[Felipe Monteiro]

Add RBAC tests for the follow test cases:

    @decorators.idempotent_id('9a36df71-a257-43a5-9555-dc7c88e66e0e')
    def test_volume_extend(self):
        # Extend Volume Test.
        volume = self.create_volume()
        extend_size = volume['size'] + 1
        self.volumes_client.extend_volume(volume['id'],
                                          new_size=extend_size)
        waiters.wait_for_volume_resource_status(self.volumes_client,
                                                volume['id'], 'available')
        volume = self.volumes_client.show_volume(volume['id'])['volume']
        self.assertEqual(volume['size'], extend_size)

    @decorators.idempotent_id('86be1cba-2640-11e5-9c82-635fb964c912')
    @testtools.skipUnless(CONF.volume_feature_enabled.snapshot,
                          "Cinder volume snapshots are disabled")
    @decorators.skip_because(bug='1687044')
    def test_volume_extend_when_volume_has_snapshot(self):
        volume = self.create_volume()
        self.create_snapshot(volume['id'])

        extend_size = volume['size'] + 1
        self.volumes_client.extend_volume(volume['id'], new_size=extend_size)

        waiters.wait_for_volume_resource_status(self.volumes_client,
                                                volume['id'], 'available')
        resized_volume = self.volumes_client.show_volume(
            volume['id'])['volume']
        self.assertEqual(extend_size, resized_volume['size'])

This should provide coverage for "volume:extend" and "volume_extension:extended_snapshot_attributes" (of course, this requires proper verification and testing).

[Felipe Monteiro (fm577c)]

Actually, these tests already exist.