Add pysaml2 to global requirements

New features for Keystone will leverage the pysaml2 library.
Ongoing patch: https://review.openstack.org/#/c/110542/
Spec:
http://specs.openstack.org/openstack/keystone-specs/specs/juno/keystone-to-keystone-federation.html
Blueprint: keystone-to-keystone-federation

Change-Id: I5d0be9d7e74b643235ea5d8f098c27be9bc775a8
Merge "Add pysaml2 to global requirements"