Remove neutron ryu-plugin support

Ryu plugin was marked deprecated in Juno and will be removed for Kilo.

We (Ryu team) recommend users to migrate to ofagent, on which
we aim to concentrate our development resources by this deprecation.

Partial-Bug: #1391714
Change-Id: I1ef28818e9400664ae3d83758dc2dcf71c02f185
Merge "Remove neutron ryu-plugin support"