Add DHCP options constants to DHCP lib
Add DHCP options constants to DHCP lib

Add DHCP options constants to DHCP lib for the following options:
* Domain name option (15)
* Interface MTU (26)
* Classless route (121)

These constants are used in Dragonflow, which uses ryu's DHCP lib to
construct a DHCP responder.

Signed-Off-By: Omer Anson <oaanson@gmail.com>

Signed-off-by: FUJITA Tomonori <93dac1fe9c4b2a3957982200319981492ad4976e@lab.ntt.co.jp>

gareging/SDN_Framework,fujita/ryu,osrg/ryu,iwaseyusuke/ryu,lagopus/ryu-lagopus-ext,shinpeimuraoka/ryu,pichuang/ryu,lagopus/ryu-lagopus-ext,fujita/ryu,lagopus/ryu-lagopus-ext,iwaseyusuke/ryu,fujita/ryu,pichuang/ryu,iwaseyusuke/ryu,osrg/ryu,iwaseyusuke/ryu,fujita/ryu,lagopus/ryu-lagopus-ext,shinpeimuraoka/ryu,osrg/ryu,pichuang/ryu,osrg/ryu,gareging/SDN_Framework,gareging/SDN_Framework,lagopus/ryu-lagopus-ext,shinpeimuraoka/ryu,fujita/ryu,shinpeimuraoka/ryu,osrg/ryu,gareging/SDN_Framework,pichuang/ryu,pichuang/ryu,shinpeimuraoka/ryu,iwaseyusuke/ryu,gareging/SDN_Framework