fix for qrewrite failing with dataset wildcards (in testbed, needs patching)

*failing query on testbed:* ```dataset dataset=/RelValZmmg*/*/* | grep dataset.nevents``` [link](https://cmsweb-testbed.cern.ch/das/request?view=list&limit=10&instance=cms_dbs_prod_global&input=dataset+dataset%3D%2FRelValZmmg*%2F*%2F*+%7C+grep+dataset.nevents)


* qrewrite was creating a DASQuery({spec: dataset.name='<PK>', ...})
    which was not passing wildcard validation (since wildcard check do not need dbsmanager to be passed).
* the simple fix is to pass a valid dataset name instead (e.g. /a/b/c)

P.S. the patch is based directly on 2.3.0 tag.
P.S. query rewrite works fine, when it is not based on dataset PK: ```run dataset=/DoubleMu/Run2012A-Zmmg-13Jul2012-v1/RAW-RECO | grep run.run_number, run.bfield```