Pin ubuntu bionic for tox-py27 and tox-py36 in no-constraints template

As per victoria cycle testing runtime and community goal[1]
we need to migrate upstream CI/CD to Ubuntu Focal(20.04).

Ubuntu Focal does not have py2.7 and py3.6 so we need to
pin the bionic nodeset for tox-py27 and tox-py36 jobs in
no-constraints template.

[1] https://governance.openstack.org/tc/goals/selected/victoria/migrate-ci-cd-jobs-to-ubuntu-focal.html

Change-Id: I4ec505dc4c89f7b03592852ceae0e596bb45937b

zuul.d/project-templates.yaml

@@ -1089,11 +1089,13 @@
     check:
       jobs:
         - tox-pep8
-        - tox-py27
+        - tox-py27:
+            nodeset: ubuntu-bionic
     gate:
       jobs:
         - tox-pep8
-        - tox-py27
+        - tox-py27:
+            nodeset: ubuntu-bionic
     post:
       jobs:
         - publish-openstack-python-branch-tarball
@@ -1121,10 +1123,12 @@
       This uses a variant that do not use constraints.
     check:
       jobs:
-        - tox-py36
+        - tox-py36:
+            nodeset: ubuntu-bionic
     gate:
       jobs:
-        - tox-py36
+        - tox-py36:
+            nodeset: ubuntu-bionic
 
 - project-template:
     name: openstack-python37-jobs-no-constraints