'cellranger mkfastq' should get local cores and memory per core from execution environment

[pjbriggs]

In the Fastq generation pipeline, when cellranger mkfastq is run in local mode then the numbers of cores and memory per core should be set from the execution environment (e.g. maximum number of available cores) rather than the numbers in the configuration (which may be more appropriate for running count or multi, and might not match the execution environment specified for mkfastq).