Fix misleading TripleO Devtest docs

The docs were misleading. People were expecting that they should ssh
into the machine as `devtest` user even when they passed in
`--extra-vars="devtest_user=root"`.

Change-Id: I6a34c860cb9d6da326ead8ef9259b4f151b9ce20
Add Tuskar development environment playbook

After running Devtest successfully, developer can run this playbook to
set up a development environment for Tuskar API + Tuskar UI +
Tuskarclient on the Undercloud machine. Docs are provided.

Change-Id: I62c22d7173e6447661c35353388c82b2f59eac5d