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/61713
  - https://gerrit.opnfv.org/gerrit/61715
  - https://gerrit.opnfv.org/gerrit/61717
  - https://gerrit.opnfv.org/gerrit/61719

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

Change-Id: I0ce45eb6c9af66546c2c020eb962fa457f6c7e1a
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>