bgpd: add [no] bgp snmp traps rfc4273 command

bgpd: add [no] bgp snmp traps rfc4273 command

There is no cli command to prevent the router to send traps
implemented in the rfc4273. If not done, when introducing
the traps from bgp4v2mib, traps will be send for each of
the two mibs: there will be redundancy in the sent information.

Add a new command:
- [no] bgp snmp traps rfc4273

Using this command will allow or not the notification of
the following traps:
- bgpEstablishedNotification
- bgpBackwardTransNotification

Signed-off-by: Francois Dumontet <francois.dumontet@6wind.com>