Update Xtesting to 0.60

Update Xtesting to 0.60

Xtesting leverages on stevedore [1] which will bring much more
flexibility (discovery, module or class renaming, etc).

OPNFV Features have to be updated as well:
  - https://gerrit.opnfv.org/gerrit/61719
  - https://gerrit.opnfv.org/gerrit/61723

[1] https://docs.openstack.org/stevedore/latest/

Change-Id: Ibe2a6ab3ddca92882aa4a26f140d2db28d77cb86
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>