Restore heat tests running with admin user, demo tenant

heat currently needs an admin user so that stacks can create users
however the tests need the demo tenant so that the neutron
private network is the default.

There is now a comment which explains the situation so that this
isn't changed again.

Change-Id: Ifd91d2a5af40e9697b51c3d48b13220d358cbcd8
Closes-Bug: #1281768