Set default for Data.publishDbsUrl.

Default Data.publishDbsUrl to 'https://cmsweb.cern.ch/dbs/prod/phys03/DBSWriter' since it is the only place where users can publish.
Accordingly, do not require Data.publishDbsUrl to be specified in the config.
Since also Data.publishDataName gets a default of <time-stamp>_crab_<requestName> if it is not specified, then I removed the requirement of having to specify Data.publishDataName in the config.