Revert "disable glance voting on py27 tests"

Previously, the glance py27 tox testenv ran both unit and functional
tests.  Change I93d7216340bef70fc18fc9943da5c740ba3d4761 separates out
the functional tests (which were causing the TIMED_OUT) from the unit
tests.  Restore the cross-glance-py27 job to run the glance unit tests
under py27.

See test patch https://review.openstack.org/#/c/536070/ to verify
successful openstack-tox-py27 run.

This reverts commit 74223de198.

Change-Id: Icb13423f83a1cc096d287b3ae3e438e0d59a024a

.zuul.d/cross-jobs.yaml

@@ -42,8 +42,6 @@
     parent: requirements-cross-test
     description: Run cross-project tests on glance with py27.
     required-projects: openstack/glance
-    # glance py27 is failing, but being activly worked on
-    voting: false
     vars:
       tox_envlist: py27
 

.zuul.d/project.yaml

@@ -63,6 +63,7 @@
         - cross-cinder-py27
         - cross-cinder-py35:
             branches: ^(?!stable/(newton|ocata)).*$
+        - cross-glance-py27
         - cross-glance-py35:
             branches: ^(?!stable/(newton|ocata)).*$
         - cross-horizon-py27