If a service is already started it will create files in the venv
__pycache__ directory. This role will then try to symlink the non empty
directory to the host, which will fail.
"the directory /openstack/venvs/nova-19.1.0.dev272/lib/python3.6/
site-packages/__pycache__ is not empty, refusing to convert it"
This patch stops any __pycache__ files from being symlinked to the
venv, these appear to be present in suse packages [1].
[1] http://paste.openstack.org/show/773974/
Change-Id: Ib718f5de4e5ae5b401858dea6437e0039b7082e0