Update NetMHC configuration handling to sync with Biokepi

[armish]

Follows up on https://github.com/hammerlab/biokepi/pull/465 and allows customizing the temporary directory that the NetMHC tools will be using via an environment variable. Also updates the biokepi_machine to match with the new signature.

With this change, secotrec-generated env file now has documentation and warning on potential issues with setting a mounted dir as the temporary dir (cf. https://github.com/hammerlab/biokepi/issues/467):

$ secotrec-gke genconf example.env
$ tail -8 example.env
# ### Variable `NETMHC_TMPDIR` (optional, default: /tmp/):
# 
#     export NETMHC_TMPDIR=/path/to/tmp/
# 
# Where the NetMHC-family of tools should store their 
# their temporary files on the pod. Note that NetMHC tools can 
# misbehave if you use a folder on one of the mounted folders.

cc: @julia326