tempest: Switch to local copy of tempset/scenario/manager.py

The manager.py file was literally copied from
tempest as of today. [1]

tempest/scenario/manager.py is not a part of tempest stable
interface for plugins. [2]

This was suggested on openstack-dev as a preparation for
the coming refactoring in tempest. [3]

As tempest is branchless, this change needs to be backported
to all relevant stable branches.

[1] d184d619d7adc6ab60055884515055a7f3cf7770
[2] https://docs.openstack.org/developer/tempest/plugin.html#stable-tempest-apis-plugins-may-use
[3] http://lists.openstack.org/pipermail/openstack-dev/2017-February/112938.html

Change-Id: I0797e2948e992fd3399e6c3683745610d0eaadff

neutron_taas/tests/tempest_plugin/tests/scenario/base.py

@@ -13,7 +13,7 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-from tempest.scenario import manager
+from neutron_taas.tests.tempest_plugin.tests.scenario import manager
 
 
 class TaaSScenarioTest(manager.NetworkScenarioTest):