Fix README images
Fix README images

Change-Id: I11282cb2ebf73cb9c833e910367962339be0508a

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