Add a metacat / data_dispatcher option equivalient for --dataset-definition

[marcmengel]

It would be --metacat-query xxx and would basically be equivalent to: -e SAM_PROJECT=$(ddisp project create xxx)
which would start the project with data_dispatcher at launch time and pass its ID in SAM_PROJECT, letting the compatability
code in ifdhc 2_7 and later use data dispatcher calls to get the next file, etc.

Users would need data_dispatcher and metacat setup to use this.

[shreyb]

Discussion points:

1. Request: Add Classad Attribute or ENV that indicates in the job which data handler we're using (SAM or Metacat). We should coordinate this with POMS, fife_wrap, fife_launch, FIFE devs.
2. We should try to isolate the metacat-specific (and while we're at it, the SAM-specific code). So our get_parser.py would, for example from metacat import BLAH) would have this arg (--metacat-query xxx`), call BLAH. The same could be done for SAM-specific options.