[OWL-678] Use `hostname -s`
[OWL-678] Use `hostname -s`

The `getHostname()` used to get the full hostname which is identical to
the result of the commandline utility `hostname`. Now it gets the string
before the character `.`.

masato25/open-falcon-backend,Cepave/open-falcon-backend,Cepave/open-falcon-backend,Cepave/open-falcon-backend,Cepave/open-falcon-backend,masato25/open-falcon-backend,masato25/open-falcon-backend,Cepave/open-falcon-backend,Cepave/open-falcon-backend,masato25/open-falcon-backend,masato25/open-falcon-backend,masato25/open-falcon-backend