Fix the docs since Federation is no longer an extension

[Rodrigo Duarte]

Currently we have a documentation to enable the Federation extension [1]. Although there is some steps that are no longer needed, some of them need to be executed in order to the functionality properly work: add the "saml2" auth method, install xmlsec1 and pysaml2. These steps should be included in the main Federation doc [2] and the extension one should be removed.

[1] https://github.com/openstack/keystone/blob/master/doc/source/extensions/federation.rst
[2] https://github.com/openstack/keystone/blob/master/doc/source/configure_federation.rst