After commit https://goo.gl/TjLQFz to pysaml2, xmldsig was moved
under saml2, no longer being at the top level. Craeting a
conditional import, so it can work now and when the pysaml2
commit is included in a new release.
Conflicts:
keystone/contrib/federation/idp.py
keystone/tests/unit/test_v3_federation.py
Change-Id: If01a93a18cef6159adad50496e6335e88f86024d
(cherry picked from commit c90dd3a0f8)
(cherry picked from commit afaaf1180b)