More compatibility with Python 3

- use the six version of configparser and urllib, and depends on six;
- remove relative imports;
- adapt few tests to the changes.

The changes above should be noop from the point of view of
functionalities, at least on python 2.

And also:
- replace the py34 tox virtualenv with py35;
- add a non-voting py35 job (locally for now, it will be enabled
  to project-config also for gating when stable).

Story: 2002574
Task: 22142
Change-Id: I0a35abaae6f5b7095ebae765fbe2163046e0a4da

config_tempest/users.py

@@ -13,7 +13,7 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
-from constants import LOG
+from config_tempest.constants import LOG
 from tempest.lib import exceptions