Implement command to change RabbitMQ log level at runtime

[lukebakken]

See this rabbitmq-users message

You can use the following command to change to debug log level at runtime:

rabbitmqctl eval '{lager_file_backend, F} = proplists:lookup(lager_file_backend, gen_event:which_handlers(lager_event)), lager:set_loglevel(lager_event, lager_file_backend, F, debug), lager:set_loglevel(rabbit_log_lager_event, lager_forwarder_backend, undefined, debug).'

This enhancement will add the set_log_level command so that the following will do the same as the above:

rabbitmqctl set_log_level debug