Change nova-next job to run with python3

The way of the future is python 3 so it makes sense
that our nova-next job, which is meant to test future
things and advanced configuration, would run under python3.

Change-Id: Ie33efb41de19837b488d175900d9a4666f073bce

.zuul.yaml

@@ -173,6 +173,7 @@
       post-test scripts to ensure those scripts are still working,
       e.g. archive_deleted_rows. In Queens, this job started testing the
       TLS console proxy code in the libvirt driver.
+      Starting in Stein, the job was changed to run with python 3.
     run: playbooks/legacy/nova-next/run.yaml
     post-run: playbooks/legacy/nova-next/post.yaml
 

playbooks/legacy/nova-next/run.yaml

@@ -24,9 +24,6 @@
       environment: '{{ zuul | zuul_legacy_vars }}'
 
     - shell:
-        # TODO(mriedem): Consider setting USE_PYTHON3=True here to make this
-        # job run under python 3.5 which is a "next" type thing.
-
         # Enable TLS between the noVNC proxy & compute nodes; this requires
         # the tls-proxy service to be enabled.
 
@@ -43,6 +40,7 @@
           NOVA_USE_SERVICE_TOKEN=True
           NOVA_CONSOLE_PROXY_COMPUTE_TLS=True
           PLACEMENT_DB_ENABLED=True
+          USE_PYTHON3=True
 
           [[post-config|$NOVA_CONF]]
           [compute]