vstart.sh: use output of hostname for cephadm

vstart.sh: use output of hostname for cephadm

Otherwise I get this error on a dev machine:

        /home/pdonnell/ceph/build/bin/ceph -c /home/pdonnell/ceph/build/ceph.conf -k /home/pdonnell/ceph/build/keyring orch host add senta03
        Error ENOENT: New host senta03 (senta03) failed check: ['INFO:cephadm:podman|docker (/bin/podman) is present', 'INFO:cephadm:systemctl is present', 'INFO:cephadm:lvcreate is present', 'INFO:cephadm:Unit chronyd.service is enabled and running', 'INFO:cephadm:Hostname "senta03" matches what is expected.', 'ERROR: hostname "senta03.front.sepia.ceph.com" does not match expected hostname "senta03"']

If `hostname` is configured to give the fqdn, we get the above error
from cephadm.

Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>