GATK: support for new gatk and gatk3 script names
GATK: support for new gatk and gatk3 script names

Preparation for new release, swaps to new script naming in GATK upstream
and bioconda as of post GATK 4.0.3.0. We can now use the latest releases.
Fixes #2430

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