Description of problem:
Currently, pulp's ContentSource class has an id field that represents source ID. In cases when the ContentSource is the primary content source, this is set to "__/primary/__".
Instead, this could just be set to "None". This would let us remove the PRIMARY_ID constant in pulp/common/constants.py.
This change is an API change so I propose that it is done with Pulp 3.0.
Version-Release number of selected component (if applicable): 2.5
Note that there is also https://bugzilla.redhat.com/show_bug.cgi?id=1135589 which moves the PRIMARY_ID definition to avoid DB initialization. The patch for that bug is more of a workaround; this BZ is to get rid of the constant entirely.
Moved to https://pulp.plan.io/issues/608