venus: mark as unbuildable as it was not released

https://review.opendev.org/c/openstack/releases/+/824394 is still open
which means Venus is not released for stable/yoga still.

Change-Id: I1f17717af38f1ad66a6a568701b6fcf5a15077fb

Author: Marcin Juszkiewicz

kolla/image/build.py

@@ -100,6 +100,7 @@ class Status(Enum):
     },
 
     'source': {
+        "venus-base",  # no stable/yoga tarballs available
     },
 
     'centos': {