Included info about Federated Identity

Updated the Security guide to include information about the overview of
Federated identity, the terms used and how Identity Service handles mapping of SAML
assertions to assigned roles.
Also included some steps for configuring Identity Service for federation.
Partial-Bug: #1319158

Change-Id: I0d0307f26935d4e5d09ea00d0f95f7e41dbfdb50
Merge "Included info about Federated Identity"