Make 'devstack-ipv6' job as voting

'devstack-ipv6' job set the devstack to deploy
the services on IPv6. As part of community goal
'Support IPv6-Only Deployments'[1], this is going
to be the base job for all project specific or tempest
IPv6 jobs. Running this as voting make sure any devstack
setting or changes would not break the IPv6 jobs.

Story: #2005477
Task: #35923

[1] https://storyboard.openstack.org/#!/story/2005477

Change-Id: Id6580e8b29b6b04e34c2c1eca3125fa08920eb1d

Author: Ghanshyam Mann

.zuul.yaml

@@ -624,8 +624,7 @@
       jobs:
         - devstack
         - devstack-xenial
-        - devstack-ipv6:
-            voting: false
+        - devstack-ipv6
         - devstack-platform-centos-7
         - devstack-platform-opensuse-150
         - devstack-platform-fedora-latest
@@ -666,6 +665,7 @@
       jobs:
         - devstack
         - devstack-xenial
+        - devstack-ipv6
         - devstack-multinode
         - devstack-multinode-xenial
         - devstack-unit-tests