Run functional job on Ubuntu Focal

Devstack is planning to remove the Ubuntu
Bionic support.
- https://review.opendev.org/c/openstack/devstack/+/788754

Before that we need to switch Bionic job to focal.
devstack-tox-functional define the latest nodeset which is
single node focal currently so removing nodeset setting from
python-cinderclient functional jobs.

Change-Id: Ibbbfb20c86bb2ea0d3d74f6a6a1bc913874f67ad

.zuul.yaml

@@ -15,7 +15,6 @@
 - job:
     name: python-cinderclient-functional-py36
     parent: python-cinderclient-functional-base
-    nodeset: openstack-single-node-bionic
     vars:
       python_version: 3.6
       tox_envlist: functional-py36
@@ -23,7 +22,6 @@
 - job:
     name: python-cinderclient-functional-py38
     parent: python-cinderclient-functional-base
-    nodeset: openstack-single-node-focal
     vars:
       python_version: 3.8
       tox_envlist: functional-py38