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
1 file changed