Installation script: --pypi-mirror, --package-mirror and --venv-mirror

Use case

Installation is pretty easy when there is an Internet connection available. And there is surely a number of OpenStack uses when whole environment is isolated. In this case, we need somehow specify where installation script should take required libs and packages.

Problem description

1. Installation script can't work without direct Internet connection

Possible solution #1

1. Add --pypi-mirror option to installation script.
2. Add --package-mirror option to installation script.
3. Add --venv-mirror option to installation script.