Fix DbsUrl for clones

Make sure the correct DbsUrl is stored in couch, even if not present in
the schema. However, if a non-null value is in the schema store that one.

The unit tests in WMCore_t.RequestManager_t.ReqMgrWorkload_t test the following cases:

- If no DbsUrl is present in the schema, then the default from the specs is stored in couchDB.
- If DbsUrl is present in the schema, then it is stored in couch regardless of the request type.
- If no DbsUrl is present in the schema, and the workload doesn't provide a default then couchDB stores a null value.

Fixes #4498
Merge pull request #4535 from dballesteros7/DBSUrl-checkup

Fix DbsUrl for clones