ReqMgr - removing wrong DBS URL from configuration

ReqMgr configuration defined:
dbs3 = 'http://vocms09.cern.ch:8989/dbs'
which is wrong. Was removed along with related code in the Announce module.
The results of handleAnnounce call here are at least questionable, pending Ops
feedback it may be removed altogether.
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 #4534 from zdenekmaxa/reqmgr-dbs3-url-removed-from-configuration

ReqMgr - removing wrong DBS URL from configuration
Merge pull request #4535 from dballesteros7/DBSUrl-checkup

Fix DbsUrl for clones
0.9.57

Signed-off-by: Zdenek Maxa <zdenek.maxa@cern.ch>