disable Servlet sessions for /restconf web context

Unless RESTCONF, or JAX RS, is doing something I am not aware of, I
doubt that we need our chosen Servlet Container (currently Jetty) tokeep
track of Servlet API HTTP Sesssion objects for connections to /restconf?

This probably saves a (little bit) of memory in Jetty & Co.

Change-Id: Ia57c22036b65b03eada81991e1566fc9da65ecb6
Signed-off-by: Michael Vorburger <vorburger@redhat.com>