README warn against virtualenv --system-site-packages

If using --system-site-packages, some system python modules may conflict
with the ones required. For example, the python modules paramiko and gssapi
conflict: if you have both, then "import paramiko" will fail while looking
for gssapi exceptions.

Also rename the sample topology to use the *.example available the
available topologies. "topology.yaml.example" does not exist.

Change-Id: I816177f305ad6f4675c12c64d49d04f075bee817
Signed-off-by: Gabriel Ganne <gabriel.ganne@enea.com>