Add mechanism to override the generated SSPDescriptor of the Federation

[laskasn]

We need to implement a mechanism to override the exported SSPDescriptor document of the SAML aggregations. Should be able to apply some sort of rules, in order to modify the exported xml document.

[laskasn]

It will be implemented with the use of XSLT snippets.
The admin will be able to define an XSLT snippet, which will be applied on-the-fly, directly before serving the xml description file.