Make multinode job voting

The multinode job is useful for catching issues on multinode
environment and this commit promote this job to voting.

In order to make the job success, this commit also fixes
two issues:
* Fix the nodeset of compute node (it should be 'subnode' instead
  of 'peers').
* Don't perform API-related configuration in compute node.

Change-Id: I16833dca7f442e5700a84dca042f04fb0edf25c7

.zuul.yaml

@@ -47,7 +47,7 @@
         KURYR_CAPABILITY_SCOPE: global
         USE_PYTHON3: true
     group-vars:
-      peers:
+      subnode:
         devstack_localrc:
           KURYR_CAPABILITY_SCOPE: global
           KURYR_PROCESS_EXTERNAL_CONNECTIVITY: false
@@ -73,7 +73,6 @@
 - job:
     name: zun-tempest-multinode-docker-sql
     parent: zun-tempest-base-multinode
-    voting: false
 
 - job:
     name: zun-tempest-docker-etcd
@@ -104,3 +103,4 @@
       jobs:
         - zun-tempest-docker-sql
         - zun-tempest-py35-docker-sql
+        - zun-tempest-multinode-docker-sql

devstack/lib/zun

@@ -120,6 +120,7 @@ function configure_zun {
     # Rebuild the config file from scratch
     create_zun_conf
 
+    if is_service_enabled zun-api; then
         create_api_paste_conf
 
         write_uwsgi_config "$ZUN_UWSGI_CONF" "$ZUN_UWSGI" "/container"
@@ -128,6 +129,7 @@ function configure_zun {
             # Switch off glance->swift communication as swift fails under py3.x
             iniset /etc/glance/glance-api.conf glance_store default_store file
         fi
+    fi
 }
 
 # upload_images() - Upload docker images to glance