bismark: don't use iGenomes index with hisat2
bismark: don't use iGenomes index with hisat2

The bismark 3-letter genome indices available on AWS iGenomes are only
built with bowtie2, causing breakage when bismark is using hisat2
for alignment. This change ensures that when bismark_hisat is chosen,
the iGenomes index is not used.

ewels/NGI-MethylSeq,ewels/NGI-MethylSeq,ewels/NGI-MethylSeq