utils: Re-implement parse_requirements
utils: Re-implement parse_requirements

This patch re-implements utils.parse_requirements() without retrieving
code from OpenStack.

Signed-off-by: IWASE Yusuke <bb80e84cc9f5a8f01385872ee89032beb5d4eba7@gmail.com>
Signed-off-by: FUJITA Tomonori <93dac1fe9c4b2a3957982200319981492ad4976e@lab.ntt.co.jp>

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