Fix image links in README
Fix image links in README

Change-Id: I651be6bb9f16311f50d5a95b4981c866423d97b5

neoareslinux/dragonflow,FrankDuan/df_code,neoareslinux/dragonflow,openstack/dragonflow,openstack/dragonflow,FrankDuan/df_code,no2key/dragonflow,no2key/dragonflow,openstack/dragonflow,FrankDuan/df_code