ci: helper: Don't make ':' literal a static part of the image tag

':' is just a connecting character, we can add it to the appropriate
place later in the Python script later, but it doesn't make sense to be
part of the image 'tag' string.

Signed-off-by: Erik Skultety <eskultet@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

ci/helper

@@ -40,7 +40,7 @@ class Parser:
         )
         containerparser.add_argument(
             "--image-tag",
-            default=":latest",
+            default="latest",
             help="use container images with non-default tags",
         )