ripd: implement the 'clear-rip-route' YANG RPC

ripd: implement the 'clear-rip-route' YANG RPC

This command deletes all received routes from the RIP routing table.
It should be used with caution as it can create black holes in the
network until RIP reconverges. Very useful to make automated testing
(e.g. ANVL) more predictable, since the internal state of ripd can be
cleared after each test.

Implement the command using a YANG RPC so that it can be executed by
other northbound clients in addition to the CLI.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>