The var tempest_plugin_install_source is defined within the
os_tempest role so if it is used in the condition that includes
the python_venv_build role, it must be in scope for the python venv
build handler which runs in the outer containing playbook, and
this fails with a var not found error.
Change-Id: I7d1fd3b7d06d3a0c6d86750ed36cbe858190f87e