vyatta-op-dhcp-server: strip shared-network prefix from client-hostname

Strip the shared-network prefix added to client-hostname in the
dhcpd.leases when running "show dhcp server leases".  This prefix was
added to fix Bug #160, to ensure unique host entries in the generated
dhcpd.conf.

Bug #160 http://bugzilla.vyos.net/show_bug.cgi?id=160