Mark the Ryu plug-in for Neutron as deprecated as of Juno

Also remove the Ryu plug-in in the list of available network plugin-ins
in the new networking guide to not document a deprecated plug-in.

Change-Id: I289c96b2aa691ccdd7197c2ecdee7697b2428ab5
Closes-Bug: #1356664
Merge "Mark the Ryu plug-in for Neutron as deprecated as of Juno"