default to old change screen

The new one lacks many very useful features and is still in beta, we should be using the old one pretty much exclusively.

Note that users can override this setting in their own preferences if they really want to use the new change screen.

playbooks/roles/gerrit/templates/gerrit.config.j2

@@ -3,7 +3,6 @@
 [gerrit]
     basePath = {{ gerrit_data_dir }}
     canonicalWebUrl = http://{{ gerrit_hostname }}/
-    changeScreen = CHANGE_SCREEN2
 [database]
     type = MYSQL
     hostname = {{ gerrit_db_hostname }}