All the platforms we care about now have python3 with venv (even
centos7 now) packaged somehow. Add an ensure-venv element to make
sure that "python3 -m venv" works. Any other elements that wish to
install non-distribution-packaged Python utilities can use this to
keep them separate from the main system installs.
Port glean to use this, and drop its dependency on pip-and-virtualenv.
Change-Id: Ic16f134fe34293bb68e7c632dd320f523366320d
