Restate installation and tutorial in terms of a virtualenv

Restate installation and tutorial in terms of a virtualenv

Since we have things like "cd myproject" in the documentation,
we are assuming some knowledge of how the project is set up.
As the vast majority of projects will have import statements
in their env.py, restate this documentation in terms of a
virtual environment where the local project is installed
in editable mode.

Change-Id: I6a11c5077c02a00145cb882b43d6c9a7c986e375
Fixes: #625