dhcp: Only send client identifier if identity_lease is enabled

The client identifier serves as unique identifier just like a unique MAC
address would, so even with identity_leases disabled some DHCP servers
might assign unique leases per identity.

testing/tests/swanctl/dhcp-dynamic/posttest.dat

@@ -3,8 +3,9 @@ dave::swanctl --terminate --ike home
 carol::systemctl stop strongswan-swanctl
 dave::systemctl stop strongswan-swanctl
 moon::systemctl stop strongswan-swanctl
-venus::cat /var/state/dhcp/dhcpd.leases
-venus::server isc-dhcp-server stop 2> /dev/null
+venus::cat /var/lib/dhcp/dhcpd.leases
+venus::service isc-dhcp-server stop 2> /dev/null
+venus::rm /var/lib/dhcp/dhcpd.leases*; touch /var/lib/dhcp/dhcpd.leases
 moon::iptables-restore < /etc/iptables.flush
 carol::iptables-restore < /etc/iptables.flush
 dave::iptables-restore < /etc/iptables.flush