Use version 2 for docker-compose for review

Also - update the test job to use xenial nodes, since that's what
our gerrit servers are. docker-compose in xenial is too old for
version 3 file format - but we didn't get an error in the test
becuase we're running on bionic.

Change-Id: I855c3df241932098c95de84cf9f92383b335702f

.zuul.yaml

@@ -1092,9 +1092,9 @@
         - name: bridge.openstack.org
           label: ubuntu-bionic
         - name: review01.openstack.org
-          label: ubuntu-bionic
+          label: ubuntu-xenial
         - name: review-dev01.opendev.org
-          label: ubuntu-bionic
+          label: ubuntu-xenial
     vars:
       run_playbooks:
         - playbooks/service-letsencrypt.yaml

playbooks/roles/gerrit/templates/docker-compose.yaml.j2

@@ -1,4 +1,4 @@
-version: '3'
+version: '2'
 
 services:
   gerrit: