labs: add regular user and tenant (demo)

This changeset creates a regular user and a tenant, both named demo.
A new demo-openstackrc.sh is added for authentication as this user.

Change-Id: I386c38ff4accc8004cc5fb84f3936565f5694683
Co-Authored-By: Pranav Salunke <dguitarbite@gmail.com>
Implements: blueprint openstack-training-labs