Add grenade-py3 job to integrated-gate-py35 template
Add grenade-py3 job to integrated-gate-py35 template

This adds the grenade-py3 job, which was introduced
in Stein and thus does not need branch restrictions
for stable branches, to the integrated-gate-py35
job template. The intent of this is to eventually
deprecate/replace the legacy integrated-gate template
for newer branches where python3 testing should be the
default.

Depends-On: https://review.openstack.org/617661

Change-Id: I7205534873f53ddc04810d9f28afb934e9f3819d

openstack-infra/openstack-zuul-jobs,openstack-infra/openstack-zuul-jobs