avocado: Simplify setting test default values

Instead of using self.params.set_default() manually for
each default value, allow test writers to set a
default_params attribute with default params, and
set them at test class init time. This way we keep
things even shorter, while allowing avocado tests
to be used stand alone.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
docs: Updating documentation to reflect default_params

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