controller dhcp-service not run when controller-dhcp-enabled=false

Currently, the default implementation for DHCP server is not using the controller (controller-dhcp-enabled=false by default). The flag only affected the rules, but didn't disable the DHCP service itself. Therefore if a DHCP packet is somehow punted to the controller, the dhcpservice will handle it and even respond. This can probably cause severe IP conflicts.

Until https://git.opendaylight.org/gerrit/#/c/44785/ is merged, this constantly happens when the DHCP packets are reaching table=46 (PNAT), and therefore punted to the controller by mistake. This causes the controller DHCP server to respond to them.

Probably need to implement the following:
1. The DHCP Server should only respond to DHCP requests punted from its table. DHCP requests arriving from other tables should be ignored.
2. The DHCP server should not run at all when the controller-dhcp-enabled flag is false)