First round of ReqMgr CouchDB values fixes.

First round of ticket 4388
https://github.com/dmwm/WMCore/issues/4388#issuecomment-15390453

Removed from CouchDB:
"ReqMgrGroupID"
"ReqMgrRequestID"
"ReqMgrRequestorID"
"ReqMgrRequestBasePriority"
"WorkflowSpec"

Unittests and reqmgr.py checks that these request parametrs never appear in
CouchDB on newly created requests.

"RequestWorkflow" is left on requests, contains URL of the spec file, though
it's redundant, parameter is however forbidden on injection and is set by
ReqMgr. Spec manipulation depends on presence of this argument. Too many
changes would be required to get rid of it. reqmgr_request:worklow would be
ported if necessary to Couch or removed completely in ReqMgr2.