Use renamed template 'integrated-gate-py3'

'integrated-gate-py35' template is going to be
renamed to 'integrated-gate-py3' in https://review.openstack.org/#/c/626078/

Integrated jobs are running on Bionic now where python 3.6 is available.
Which means gate jobs in 'integrated-gate-py35' template are
running on python 3.6 not on 3.5 which makes this template name confusing.

depends on commit rename the 'integrated-gate-py35' to 'integrated-gate-py3'
so that it can convey that template will use available python 3 version
in used distro. For example: 3.5 in xenial and 3.6 in bionic and so on.

This commit starts using the new template name so that old
template name can be removed.

Depends-On: https://review.openstack.org/#/c/626078/
Change-Id: Ida85de2214cc29b973e022770ccd0f174df838ad

.zuul.yaml

@@ -191,7 +191,7 @@
     templates:
       - check-requirements
       - integrated-gate
-      - integrated-gate-py35
+      - integrated-gate-py3
       - openstack-lower-constraints-jobs
       - openstack-python-jobs
       - openstack-python35-jobs