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/ikev2/dhcp-static-client-id/hosts/moon/etc/strongswan.conf

@@ -6,6 +6,7 @@ charon {
   plugins {
     dhcp {
       server = 10.1.255.255
+      identity_lease = yes
     }
   }
 }