Fix py27 and py35 zuul jobs failing to setup

As of March 1, when jobs run on ubuntu-xenial, they fail
to run py27 and py35 jobs with an error similar to:

/usr/bin/python3 -m virtualenv --no-download
  --python /usr/bin/python2.7 py27

ImportError: cannot import name 'ContextManager'

It does not appear to impact the ubuntu-bionic nodes so
updating zuul to use those nodes.

bionic nodes do not support python3.5, so the py35 jobs
is now converted to py36.

Partial-Bug: 1865554
Change-Id: Iad13d03ced21294d1a02c82f4e1e37cac3f27ee7
Signed-off-by: albailey <Al.Bailey@windriver.com>

.zuul.yaml

@@ -12,13 +12,13 @@
         - openstack-tox-pep8
         - stx-fault-build
         - fault-rest-api-py27
-        - fault-rest-api-py35
+        - fault-rest-api-py36
     gate:
       jobs:
         - openstack-tox-linters
         - openstack-tox-pep8
         - fault-rest-api-py27
-        - fault-rest-api-py35
+        - fault-rest-api-py36
     post:
       jobs:
         - stx-fault-upload-git-mirror
@@ -62,7 +62,7 @@
     parent: tox
     description: |
       Run py27 test for fm-rest-api
-    nodeset: ubuntu-xenial
+    nodeset: ubuntu-bionic
     required-projects:
       - starlingx/config
     vars:
@@ -70,15 +70,15 @@
       tox_extra_args: -c fm-rest-api/fm/tox.ini
 
 - job:
-    name: fault-rest-api-py35
+    name: fault-rest-api-py36
     parent: tox
     description: |
-      Run py35 test for fm-rest-api
-    nodeset: ubuntu-xenial
+      Run py36 test for fm-rest-api
+    nodeset: ubuntu-bionic
     required-projects:
       - starlingx/config
     vars:
-      tox_envlist: py35
+      tox_envlist: py36
       tox_extra_args: -c fm-rest-api/fm/tox.ini
 
 - job:

fm-rest-api/fm/tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py27,py35
+envlist = py27,py36
 minversion = 2.3
 skipsdist = True
 stxdir = {toxinidir}/../../../
@@ -23,8 +23,8 @@ commands =
   stestr run {posargs}
   stestr slowest
 
-[testenv:py35]
-basepython = python3.5
+[testenv:py36]
+basepython = python3.6
 commands =
   stestr run {posargs}
   stestr slowest