Copy cfg and iniparser from oslo-incubator

https://review.openstack.org/20753 introduced tools/install_venv_common.py
but not it's dependencies openstack.common.{cfg,iniparser}

Change-Id: I270a1d8f6fd8f93988a5e2ccbc446adda0a6cd81
Merge "Copy cfg and iniparser from oslo-incubator"