Add virtual env builder

tools/build_venv.sh:
* Install required system packages (python dev, virtualenv)
* Create a virtualenv, including the contents of files/venv-requirements.txt
* Install common packages that have system dependencies

Other changes to DevStack:
* Add USE_VENV and VENV_PATH to screenrc
* Create venv early in the stack.sh run
* Change pip_install to not use sudo if it detects a virutal env is
  present ($VIRTUAL_ENV is set)
* Return the venv path in get_python_exec_prefix() when a venv is active
* pip_install() does not need to be root when a venv is active
* nova-rootwrap needed some PATH help in sudo

Change-Id: I616f405c15086820d8e158c2b046af8f417049f4
Merge branch 'screen-logdir' into dt-test
Merge branch 'identity-v3-default' into dt-test
Merge branch 'conf-tempest' into dt-test
Merge branch 'test-service-functions' into dt-test
Merge branch 'exercise-refresh' into dt-test