/usr/sbin/rabbitmq-plugins is in rabbitmq-server 3.5.7-1ubuntu0.16.04.2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/sbin/../lib/rabbitmq/bin/rabbitmq-script-wrapper
/usr/sbin/../lib/rabbitmq/bin/rabbitmq-script-wrapper