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/services/image.py

@@ -15,12 +15,13 @@
 
 import os
 import shutil
-import urllib2
 
-from base import VersionedService
-from config_tempest.constants import LOG
+from six.moves import urllib
 from tempest.lib import exceptions
 
+from config_tempest.constants import LOG
+from config_tempest.services.base import VersionedService
+
 
 class ImageService(VersionedService):
 
@@ -173,7 +174,7 @@ class ImageService(VersionedService):
             LOG.info("Image '%s' already fetched to '%s'.", url, destination)
             return
         LOG.info("Downloading '%s' and saving as '%s'", url, destination)
-        f = urllib2.urlopen(url)
+        f = urllib.request.urlopen(url)
         data = f.read()
         with open(destination, "wb") as dest:
             dest.write(data)