avocado.aexpect: Don't use a predictable, non system wide tmp dir

Instead of using /tmp/aexpect, use per aexpect instance
directories, tied to their aexpect ids (that are randomly
generated). This way we fix the potential security issue,
and also, clean up more efficiently the aexpect tmp files
(all is gone upon the .close() method happens, or garbage
collection - whatever happens first).

Put a DEBUG flag on the aexpect module, that will leave
the tmp files in case people want to debug the code.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>