Switch testing to Yoga testing runtime

Updating the testing template to Yoga testing runtime:
https://governance.openstack.org/tc/reference/runtimes/yoga.html

The commmit also removes refstack-tox-pip-check-reqs job due to a
bug in the pip-check-reqs tool:
https://github.com/r1chardj0n3s/pip-check-reqs/issues/70
and the fact the tool is not used in the gates by openstack
projects - still can be executed manualy though.

Change-Id: I6d64cb67529585b458b28ed4fe011d50d1f23d6d

.zuul.yaml

@@ -3,12 +3,11 @@
       - nodejs6-jobs
       - publish-openstack-docs-pti
       - openstack-cover-jobs
-      - openstack-python3-xena-jobs
+      - openstack-python3-yoga-jobs
     check:
       jobs:
         - refstack-tox-functional:
             nodeset: openstack-single-node-bionic
-        - refstack-tox-pip-check-reqs
     gate:
       jobs:
         - refstack-tox-functional:
@@ -26,16 +25,3 @@
       - ^releasenotes/.*$
     vars:
       tox_envlist: functional
-
-- job:
-    name: refstack-tox-pip-check-reqs
-    parent: openstack-tox
-    description: |
-      Run pip-extra-reqs and pip-missing-reqs to find any missing or redundant
-      dependencies. Uses tox with the ``pip-check-reqs`` environment.
-    irrelevant-files:
-      - ^.*\.rst$
-      - ^doc/.*$
-      - ^releasenotes/.*$
-    vars:
-      tox_envlist: pip-check-reqs