Move tempest-tox-plugin-sanity-check to Bionic

This commit makes tempest-tox-plugin-sanity-check job
to run on Bionic. This job only run on master Tempest gate
so no need to worry about its xenial version.

Change-Id: I6d8c63a62416b35e9d203cbeabc9868aa6a95f44
diff --git a/.zuul.yaml b/.zuul.yaml
index edaa959..cf1c7c4 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -346,7 +346,7 @@
     parent: tox
     description: |
       Run tempest plugin sanity check script using tox.
-    nodeset: ubuntu-xenial
+    nodeset: ubuntu-bionic
     vars:
       tox_envlist: plugin-sanity-check
     voting: false