Add an identity user guide to replace the current placeholder. Also,
add identity examples and related tests to go along with the guide.
Much of the content for the guide was pulled from [1].
[1] http://docs.openstack.org/admin-guide-cloud/identity_management.html
Change-Id: Iec3caf601732c5c6f63331eac770d1325051aa82
Closes-Bug: #1466182