allow user to manually set desired hostname 

Hi, attached patch attempts to fix an issue (specially inside docker containers) where too often logstash-forwarder relays data with the short hostname as source (an alternative approach would be to just actually call hostname -f but here i kept using exclusively golang machinery in order to keep platform portability).
I think it's clearer if it just says "Hostname detection failed. Set -hostname flag or fix this server's hostname reporting" maybe?