vdsm.spec: conditional for hostname require

Fedora rawhide (F21) build is failing because cannot locate /bin/hostname.
It has changed to /usr/bin/hostname as changelog from hostname 3.14-2.

Koji info:
http://koji.fedoraproject.org/koji/buildinfo?buildID=471302

Change-Id: I865d1b84fe646932d3039b131bc236d36b8550ba
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21194
Reviewed-by: Bala.FA <barumuga@redhat.com>
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>