Mocking ryu plugin notifier in ryu plugin test

Previously RPC notifier is not mocked in ryu plugin test and the
plugin tries to send RPC notifications. It may interact with
impl_fake RPC set up in previous tests by other plugin
and sometimes leads to unexpected ovs-vsctl invoking.

Change-Id: I11714a160faa4d3ded7f312c39afba0b426ccd8a
Closes-Bug: #1255421
(cherry picked from commit dc2f567df3972465797d4d5b492ac1c243737cfe)