Enable multicore pbgzip if specified in tools_on
Enable multicore pbgzip if specified in tools_on

Correctly assigns cores to make use of pbgzip if present in `tools_on:
[pbgzip]`. Previously we only gave each pbgzip process per fastq a
single core, this now assigns all available cores. Improves speed of
fastq preparation.

gifford-lab/bcbio-nextgen,biocyberman/bcbio-nextgen,lbeltrame/bcbio-nextgen,a113n/bcbio-nextgen,mjafin/bcbio-nextgen,Cyberbio-Lab/bcbio-nextgen,chapmanb/bcbio-nextgen,gifford-lab/bcbio-nextgen,vladsaveliev/bcbio-nextgen,mjafin/bcbio-nextgen,brainstorm/bcbio-nextgen,vladsaveliev/bcbio-nextgen,lbeltrame/bcbio-nextgen,Cyberbio-Lab/bcbio-nextgen,brainstorm/bcbio-nextgen,mjafin/bcbio-nextgen,chapmanb/bcbio-nextgen,biocyberman/bcbio-nextgen,biocyberman/bcbio-nextgen,gifford-lab/bcbio-nextgen,a113n/bcbio-nextgen,chapmanb/bcbio-nextgen,lbeltrame/bcbio-nextgen,vladsaveliev/bcbio-nextgen,Cyberbio-Lab/bcbio-nextgen,brainstorm/bcbio-nextgen,a113n/bcbio-nextgen