relay: allow override of hostname

Based on commit
https://github.com/ianslai/carbon-c-relay/commit/7edc98e55fe0ac5139a342d1435b750825761ec5
by Ian Lai, simply allow overriding the hostname as determined by
gethostname, such that it can be set to something which makes sense in
case it fails, or when sysadmins have set some undesirable name, or when
there simply is a need to change it to something which makes more sense
in the environment it runs in.