avoid oslo.config 3.1.0 and later for python2.6 support
avoid oslo.config 3.1.0 and later for python2.6 support

oslo.config v3.1.0 dropped python2.6 support:

https://github.com/openstack/oslo.config/commit/9470f19eacb99425055946bd4a6079b71406ff1b

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

gareging/SDN_Framework,OpenState-SDN/ryu,osrg/ryu,gareging/SDN_Framework,TakeshiTseng/ryu,pichuang/ryu,hisaharu/ryu,OpenState-SDN/ryu,fkakuma/ryu,shinpeimuraoka/ryu,habibiefaried/ryu,hisaharu/ryu,pichuang/ryu,Tejas-Subramanya/RYU_MEC,takahashiminoru/ryu,pichuang/ryu,fujita/ryu,hisaharu/ryu,fujita/ryu,zangree/ryu,lagopus/ryu-lagopus-ext,zangree/ryu,shinpeimuraoka/ryu,Zouyiran/ryu,gareging/SDN_Framework,Tejas-Subramanya/RYU_MEC,alanquillin/ryu,iwaseyusuke/ryu,iwaseyusuke/ryu,fujita/ryu,osrg/ryu,Zouyiran/ryu,fkakuma/ryu,zangree/ryu,Tejas-Subramanya/RYU_MEC,takahashiminoru/ryu,iwaseyusuke/ryu,osrg/ryu,alanquillin/ryu,Zouyiran/ryu,habibiefaried/ryu,lagopus/ryu-lagopus-ext,pichuang/ryu,takahashiminoru/ryu,lagopus/ryu-lagopus-ext,OpenState-SDN/ryu,osrg/ryu,alanquillin/ryu,iwaseyusuke/ryu,Zouyiran/ryu,shinpeimuraoka/ryu,fkakuma/ryu,OpenState-SDN/ryu,pichuang/ryu,Tejas-Subramanya/RYU_MEC,Tejas-Subramanya/RYU_MEC,OpenState-SDN/ryu,takahashiminoru/ryu,lagopus/ryu-lagopus-ext,fujita/ryu,gareging/SDN_Framework,TakeshiTseng/ryu,osrg/ryu,shinpeimuraoka/ryu,fkakuma/ryu,zangree/ryu,hisaharu/ryu,zangree/ryu,lagopus/ryu-lagopus-ext,alanquillin/ryu,Zouyiran/ryu,alanquillin/ryu,takahashiminoru/ryu,shinpeimuraoka/ryu,fkakuma/ryu,fujita/ryu,TakeshiTseng/ryu,habibiefaried/ryu,habibiefaried/ryu,TakeshiTseng/ryu,gareging/SDN_Framework,iwaseyusuke/ryu,hisaharu/ryu,habibiefaried/ryu,TakeshiTseng/ryu