JobDB.update() selection by hostname doesn't work on all systems

[bpuchala]

From jobdb.py:

# Parse our hostname so we can only select jobs from THIS host
#   Otherwise, if we're on a multiple-clusters-same-home setup,
#   we may incorrectly update jobs from one cluster onto the other

This is using socket.gethostname() to create a hostname, which then is used in a regex to select which jobs to update. This doesn't work on all systems. Thoughts @tallakahath?

My first thought is to check for a CASM_PBS_JOB_UPDATE environment variable, which if either unset or set to "DEFAULT" (upper or lowercase) will not check for hostname, or if set to "CHECK_HOSTNAME" will select by hostname. I don't plan it now, but we could extend this to enable full customization with something like: "CUSTOM MODULE FUNCTION" in which case the specified module & function would be loaded and used.