Try to get best parallel testing values

Try to get best parallel testing values

It's no simple to get the best threads (test cases and tempest) values
and then guarantee Functest vs:
  - test flapping
  - backend limitations

As opposed to the OpenStack gates, Jenkins runs all test cases in
parallel which indirectly increases the number of threads.

Let's run smoke sequentially which is closed to the OpenStack model.

Change-Id: I6172568b915d49e2723173b7c428970110f63620
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>