avatar: use a custom venv directory

This change:
* Remove dependency over `virtualenv`.
* Improve TF `PythonVirtualenvPreparer` step performance (only first run
  perform a complete install).

Also:
* Force use of `python3`.
* Fixed bad default testbed name is usage.

Test: avatar run
Change-Id: I50c69108934b017651ac5e433c5d84a6a53c6155