[stable-only] Set nova-tox-functional-centos8-py36 non-voting

The centos8 py36 functional job started to fail in zuul with:

ERROR: Could not find a version that satisfies the requirement
  futures>=3.0; python_version == "3.6" (from pypowervm) (from
  versions: 0.2.python3, 0.1, 0.2, 1.0, 2.0, 2.1, 2.1.1, 2.1.2,
  2.1.3, 2.1.4, 2.1.5, 2.1.6, 2.2.0, 3.0.0, 3.0.1, 3.0.2, 3.0.3,
  3.0.4, 3.0.5, 3.1.0, 3.1.1)
ERROR: No matching distribution found for futures>=3.0;
  python_version == "3.6"

This patch sets nova-tox-functional-centos8-py36 as non-voting
to unblock the gate. This patch should be reverted as soon as
there will be a fix for this error. This is only needed in Xena.

Change-Id: Iaf389a12fdd2f27fe89a338e499837165e229095

Author: Elod Illes

.zuul.yaml

@@ -28,6 +28,9 @@
       zuul_work_dir: src/opendev.org/openstack/nova
       bindep_profile: test py36
     timeout: 3600
+    # NOTE(elod.illes): this job started to fail in stable/xena so let's
+    # set it non-voting to unblock the gate.
+    voting: false
 
 - job:
     name: nova-tox-functional-py38