GATK: don't assume gatk bioconda wrapper present
GATK: don't assume gatk bioconda wrapper present

Avoids confusing errors if bioconda `gatk` wrapper not available on
a machine. Thanks to @bwubb

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