Run jobs on bionic instead of xenial

We want to migrate to the latest Ubuntu LTS release 18.04 aka Bionic.
This is just a patch to find possible issues before the real migration
starts. See [0] for the big picture.

Also test running tox jobs on Bionic.

[0] https://etherpad.openstack.org/p/devstack-bionic

Depends-On: https://review.openstack.org/610977
Change-Id: I748f26faf6f98e9693b5cd803577df1118121b41

.zuul.d/base.yaml

@@ -32,10 +32,10 @@
         label: centos-7
 
 - nodeset:
-    name: kolla-ubuntu
+    name: kolla-ubuntu-bionic
     nodes:
       - name: primary
-        label: ubuntu-xenial
+        label: ubuntu-bionic
 
 - job:
     name: kolla-base

.zuul.d/ubuntu.yaml

@@ -31,7 +31,7 @@
 - job:
     name: kolla-build-ubuntu-binary
     parent: kolla-base
-    nodeset: kolla-ubuntu
+    nodeset: kolla-ubuntu-bionic
     voting: false
     vars:
       action: build
@@ -50,7 +50,7 @@
 - job:
     name: kolla-build-ubuntu-source
     parent: kolla-base
-    nodeset: kolla-ubuntu
+    nodeset: kolla-ubuntu-bionic
     vars:
       action: build
       base_distro: ubuntu