API selftest: move from adhoc parameters to standard (multiplex based) parameters

It makes sense to reuse the Avocado standard (multiplex based) parameters instead
of hardcoded parameters. This allows to run tests to remote servers by editing
the `api.yaml` only and not the test itself.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Merge pull request #8 from clebergnu/api_selftests_params

API selftest: move from adhoc parameters to standard (multiplex based) p...