Namespace Zuul jobs and use ubuntu bionic

The xenial image that was used by tox-linters had a version of
python not high enough for black (>=3.6).
Let's test everything on bionic and ensure that we control which nodeset
are used.

Change-Id: Ifb65cef05abcd67fba7f0941cf313dfc23e6b3ca

.zuul.d/jobs.yaml

@@ -1,15 +1,24 @@
 - job:
     name: ara-server-ansible-integration
     parent: tox
+    nodeset: ubuntu-bionic
     vars:
       tox_envlist: ansible-integration
 
 - job:
-    name: tox-py3
+    name: ara-tox-py3
     parent: tox
+    nodeset: ubuntu-bionic
     vars:
       tox_envlist: py3
 
+- job:
+    name: ara-tox-linters
+    parent: tox
+    nodeset: ubuntu-bionic
+    vars:
+      tox_envlist: linters
+
 # TODO: Delete this, all linters have been merged into tox-linters
 - job:
     name: tox-format

.zuul.d/project.yaml

@@ -7,9 +7,9 @@
       jobs:
         - ara-server-ansible-integration:
             voting: false
-        - tox-linters
+        - ara-tox-linters
-        - tox-py3
+        - ara-tox-py3
     gate:
       jobs:
-        - tox-linters
+        - ara-tox-linters
-        - tox-py3
+        - ara-tox-py3