Multiple IdPs problem

The documentation is modified in order to allow mulitple
IdPs using the OS-Federation avoiding conflicts among them.

The previous proposed configuration allows user from one IdP
to get mapped as user from a different IdP. With the chenge
proposed this is not anymore possible.

Change-Id: I9d62a840c122fb36c02c56a84c4f2ef8c30303c4
Closes-Bug: 1390124