Update centos 8 py36 functional job nodeset to centos stream 8

centOS 8 image and nodeset is planned to be remove as centOS is
EOLing soon.
- http://lists.openstack.org/pipermail/openstack-discuss/2022-January/026621.html

Devstack is also removing devstack-single-node-centos-8 nodeset
which is used in nova py36 functional job
- https://review.opendev.org/q/I36751569d92fbc5084b8308d423a75318ae7d406

This commit updates the nodeset in nove jobs to
devstack-single-node-centos-8-stream which will run functional
test on py36 env.

Change-Id: I50ced59e3aa86e629702f0fece2c4cb990ebb1ed
(cherry picked from commit b88ca72)

Author: gmaanos

.zuul.yaml

@@ -4,7 +4,7 @@
 - job:
     name: nova-tox-functional-centos8-py36
     parent: openstack-tox-functional-py36
-    nodeset: devstack-single-node-centos-8
+    nodeset: devstack-single-node-centos-8-stream
     description: |
       Run tox-based functional tests for the OpenStack Nova project
       under cPython version 3.6 with Nova specific irrelevant-files list.