Pass --fqdn to ssh hostname checks

The cluster verify checks for fqdn are done via address lookups, and
there we actually use the FQDN. However, for the ssh hostname check
which is done at node add time, we rely on the default of the “hostname”
command. And Debian for example recently changed the default to return
the shortname unless one passes ‘--fqdn’.

This patch is imported from the Debian packaging.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>