Switch default nodeset to ubuntu-bionic

Following the plan described in
http://lists.openstack.org/pipermail/openstack-discuss/2019-March/003584.html
the default node type is changed to ubuntu-bionic. Projects which
need to use other nodesets on different branches for certain jobs
must add matchers for those branches to override them accordingly.

Depends-on: https://review.openstack.org/641895

Change-Id: Ib599e17e97abab5dc1f122860126c1b1061b47e1

zuul.yaml

@@ -112,8 +112,8 @@
     post-timeout: 1800
     nodeset:
       nodes:
-        - name: ubuntu-xenial
+        - name: ubuntu-bionic
-          label: ubuntu-xenial
+          label: ubuntu-bionic
     secrets:
       - site_logs