Run tox-py35 jobs on xenial

The tox-py35 job now uses bionic nodes as default which doesn't
include python 3.5. Pin the nodeset to xenial to fix this for nodepool.
While at it the nodeset of the py36 jobs now can be dropped because
it's already the default.

Change-Id: Ie05225e791b3263f1130f5c63d9c4c31f484d70a

.zuul.yaml

@@ -212,9 +212,9 @@
       jobs:
         - tox-docs
         - tox-pep8
-        - tox-py35
-        - tox-py36:
-            nodeset: ubuntu-bionic
+        - tox-py35:
+            nodeset: ubuntu-xenial
+        - tox-py36
         - nodepool-zuul-functional:
             voting: false
         - nodepool-functional-py35:
@@ -229,9 +229,9 @@
       jobs:
         - tox-docs
         - tox-pep8
-        - tox-py35
-        - tox-py36:
-            nodeset: ubuntu-bionic
+        - tox-py35:
+            nodeset: ubuntu-xenial
+        - tox-py36
         - zuul-quick-start
         - nodepool-upload-image
     post: