CWL: ensure genome_context is list for single item
CWL: ensure genome_context is list for single item

Handles cases like hg38 where we only have one context file for
post-validation stratification.

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