WDL: Add placeholder for reading variable outputs
WDL: Add placeholder for reading variable outputs

Adds general idea for reading outputs from standard CWL json files
and initial work on validating WDL output.

brainstorm/bcbio-nextgen,chapmanb/bcbio-nextgen,a113n/bcbio-nextgen,brainstorm/bcbio-nextgen,biocyberman/bcbio-nextgen,lbeltrame/bcbio-nextgen,lbeltrame/bcbio-nextgen,a113n/bcbio-nextgen,chapmanb/bcbio-nextgen,biocyberman/bcbio-nextgen,a113n/bcbio-nextgen,biocyberman/bcbio-nextgen,brainstorm/bcbio-nextgen,vladsaveliev/bcbio-nextgen,vladsaveliev/bcbio-nextgen,chapmanb/bcbio-nextgen,vladsaveliev/bcbio-nextgen,lbeltrame/bcbio-nextgen