avocado.test: Add convenience accessors to self.params

Make it possible to access param keys through attributes:

    self.params.sleep_length

instead of:

    self.params.get('sleep_length')

Also, introduce a params.set_default() method, that allows
to provide defaults to a given param attribute, in case
the configuration did not provide one (avocado tests running
as stand alone unittests).

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
tests/sleeptest: Make use of the new accessors

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