This is a fix for Id8347b6b09735659a7ed9bbe7f9d2798fbec9620 which did
not specify the full path in ensure_pip_virtualenv_command for Xenial.
This slipped by testing because there we check
ensure_pip_virtualenv_command runs under a shell:, but not when called
as the argument to the pip: module (which exec's it differently and
requires the full path). Update testing to do that too.
Change-Id: I65ff5ce913917079ab2fc1d88c56d1c0a24ea83e
f578a38693