yang: add a module translator for ietf-rip.yang

yang: add a module translator for ietf-rip.yang

As it can be seen below, this translator covers only ~13% of the
ietf-rip YANG module. Work must be done to increase that number.

ripd> en
ripd# conf t
ripd(config)# yang module-translator load /tmp/frr-ietf-translator.json
% Module translator "ietf" loaded successfully.

ripd(config)# do show yang module-translator
 Family  Module           Deviations                      Coverage (%)
 -----------------------------------------------------------------------
 ietf    ietf-interfaces  frr-deviations-ietf-interfaces  3.92
 ietf    ietf-routing     frr-deviations-ietf-routing     1.56
 ietf    ietf-rip         frr-deviations-ietf-rip         13.60

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