Use Python 3.7 for setting up DevStack on CentOS

Last Nova has stopped supporting python versions smaller
than version 3.6. This is creating troubles to use
CentOS 7 with devstack because support for Python 3 and
CentOS 7 is still not merged

This has been required as below change is still not merged:
 - https://review.opendev.org/688799/

Change-Id: Ia3a15b8fe2badbcb0831136a85eeddc7c65fb524

playbooks/tobiko-devstack-pre-run.yaml

@@ -35,10 +35,21 @@
 
 - hosts: all
   roles:
+
+    - name: ensures has 'python3' and 'pip3' commands
+      role: python
+      vars:
+        python_command: python3
+        python_version: 3.7
+        python_release: 3.7.5
+        pip_command: pip3
+
     - role: copy-build-sshkey
       copy_sshkey_target_user: stack
-    - multi-node-setup
+
-    - orchestrate-devstack
+    - role: multi-node-setup
+
+    - role: orchestrate-devstack
 
 
 - hosts: tempest

zuul.d/tobiko-devstack.yaml

@@ -218,16 +218,6 @@
     parent: tobiko-devstack-faults
     nodeset: openstack-three-node-centos-7
     voting: false
-    vars:
-      devstack_localrc:
-        # CentOS has no support for Python3
-        USE_PYTHON3: False
-      python_version: 2
-    group-vars:
-      subnode:
-        devstack_localrc:
-          USE_PYTHON3: False
-        python_version: 2
 
 
 - job: