Fix setup_env "Could not find .egg-info directory" errors.
Fix setup_env "Could not find .egg-info directory" errors.

Recently, pip install Tempest required packages results in
"Could not find .egg-info directory" errors for a number of the
pacakages (wrapt, positional ..).  This patch upgrades "virtualenv"
to version 15.0.3 which includes newer "setuptools" and "pip" that
would fix the .egg-info errors.

Change-Id: I098df3e8118782713e930fbd4c52a904c96045bc

stackforge/refstack-client,openstack/refstack-client,stackforge/refstack-client,openstack/refstack-client