Revert "venus: mark as unbuildable as it was not released"

Reason for revert: venus was not released for yoga and thus isn't
available for this branch

This reverts commit 2db1201.

Change-Id: I83ab903fa74bbb6e963c0f05e4be8792f0253f3b

Author: osfrickler

kolla/image/build.py

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