Tweaks to make virtual environment provisioning work.
- Specify virtualenv command with --no-download (avoids updating pip and setuptools on creation) - Update pip to use --no-index (does not support --no-download) - Use system packages (needed for e.g. pyyaml which is installed as a deb and not in the dependencies dir)
Merged from https://code.launchpad.net/~roadmr/canonical-identity-provider/sso-charm-virtualenv-galore/+merge/427122