(Re-)enable vnc console tests in nova-multi-cell job

This reverts commit 47b73030cb.

The dependent devstack change fixes the issue so we can enable
vnc console tempest testing in the nova-multi-cell job.

Depends-On: https://review.opendev.org/661391/

Change-Id: Iad71b6e15bb91e51080cd9f940410dc9af791963
Related-Bug: #1830417

.zuul.yaml

@@ -267,8 +267,6 @@
               # allow_resize_to_same_host=True cold migrate will try to migrate
               # on the same host which is not supported by the libvirt driver.
               cold_migration: false
-              # NOTE(mriedem): Skip novnc tests until bug 1830417 is fixed.
-              vnc_console: false
       devstack_services:
         # Disable other non-essential services that we don't need for this job.
         c-bak: false