Get rid of 'profiles' in module_defaults association

[JayZ12138]

Previously, 'profiles' was the key part to get the right modulemd_defaults
units to associate and unassociate. However, as the keyword in profiles
always include speical character '.'(dot), it leads to mongodb query failure.

Since while assmbling the lists of md_defaults_assoc and md_defaults_unassoc,
the 'profiles' has already been considered, we don't have to re-query with
it, but, because association and unassociation happens at the same time in
threads, it could delete the new unit if association happens before unassociation,
so, we serialize the modulemd_defaults units unassociation and association.

[rbikar]

1. Please add test coverage.
2. Isn't there a way how to pass special chars to mongo query?
3. Otherwise it looks ok.

[JayZ12138]

1. Please add test coverage.
2. Isn't there a way how to pass special chars to mongo query?
3. Otherwise it looks ok.

1. Yes, I'm adding
2. No. There isn't any way to pass '.' to query in keyword. The mongodb team is trying to support it.

[rohanpm]

2. No. There isn't any way to pass '.' to query in keyword. The mongodb team is trying to support it.

That's amazing. I almost couldn't believe it, but it's right there in
https://docs.mongodb.com/manual/reference/limits/#Restrictions-on-Field-Names, "The MongoDB Query Language cannot always meaningfully express queries over documents whose field names contain these characters". So Pulp is saving modulemd_defaults units which can't possibly be searched over, it's crazy.

already resolved