Hardcode service setup python interpreter

Ansible discovers CentOS 8 ansible_python['executable'] as /usr/libexec/platform-python

While it's system python which has all modules it might have side effects
Moreover, we can't rely that installed python extra binaries will be
available in the same dir as ansible_python['executable'] is.

In the meanwhile `/usr/bin/python3` is valid python path for all
currently supported distros.

Change-Id: I198bf4b23dd7b120889e65cc8b16f8e8ca78b892

playbooks/defaults/distro_install.yml

@@ -72,4 +72,4 @@ openstack_db_setup_host: "{{ groups['utility_all'][0] }}"
 openstack_db_setup_python_interpreter: "{{ ansible_python['executable'] }}"
 
 openstack_service_setup_host: "{{ groups['utility_all'][0] }}"
-openstack_service_setup_host_python_interpreter: "{{ ansible_python['executable'] }}"
+openstack_service_setup_host_python_interpreter: "/usr/bin/python3"

zuul.d/jobs.yaml

@@ -467,7 +467,6 @@
     name: openstack-ansible-deploy-aio_distro_metal-centos-8
     parent: openstack-ansible-deploy-aio-distro
     nodeset: centos-8
-    voting: false
 
 - job:
     name: openstack-ansible-deploy-aio_nspawn-centos-8