Use same hostname function as nova

Use same hostname function as nova

Certain plugin implementations of port binding depend on the port's
binding:host_id set by nova matching the compute node's hostname
reported by neutron agents in agents_db. Nova uses
socket.gethostname() to obtain the hostname, so this changes neutron
to also use socket.gethostname(), instead of socket.getfqdn() to
obtain the default hostname. If deployments require a specific
hostname format or value, the hostname can be specified in nova and
neutron configuration files.

Closes-Bug: 1223000
Change-Id: I5de9c6b0df16eca482037a5bdc3a77dc52f4adba