CNVkit: improve parallelization single machines
CNVkit: improve parallelization single machines

Transfer for using CNVkit parallelization on coverage and segmentation
instead of bcbio native. Enables better core usage and parallelization
on single machine runs since we can re-parallelize with multicore
when calling out to CNVkit. #1695 #1582

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