Change test-requirements.txt ajkavanagh to openstack

Changed from:
git+https://.../ajkavanagh/charms.openstack.git#egg=charms-openstack
to:
git+https://.../openstack/charms.openstack.git#egg=charms-openstack

This is because the tests should come from the openstack master branch.

test-requirements.txt

@@ -3,4 +3,4 @@ os-testr>=0.4.1
 charms.reactive
 mock>=1.2
 coverage>=3.6
-git+https://github.com/ajkavanagh/charms.openstack.git#egg=charms-openstack
+git+https://github.com/openstack/charms.openstack.git#egg=charms-openstack