I like the criteria based approach but after using it I think we should offer a shortcut that explicity takes a repo ID. That argument will stuff the ID into the appropriate place and format in the criteria, but it makes the CLI a bit cleaner.

For instance, I went to add a single repo to a group:

$ pulp-admin repo group members add --group-id comics --str-eq "id=avengers"

I think we can offer a simpler shortcut for this:

$ pulp-admin repo group members add --group-id comics --repo-id avengers

The server-side doesn't have to change, the client just takes the ID and adds it to the criteria as appropriate. I also suggest we make this argument allowable to be specified multiple times, that way I can do:

$ pulp-admin repo group members add --group-id comics --repo-id avengers --repo-id jla

I figure in most cases people will want to add a single repo so it might be nice to offer a simpler way of doing that.