Enable dump/restore users from RabbitMQ ver. 3.6.x

RabbitMQ changed internal_users scheme since ver. 3.6.0. See the
following links for further details:

• rabbitmq/rabbitmq-server#270
• rabbitmq/rabbitmq-server#310
• rabbitmq/rabbitmq-common@9c86a7401cf464dc20527890192c5dc0fe43b6c8
• rabbitmq/rabbitmq-server@93b5a3a8092f52063cbca3ab661c7c6bae43c512

CC @oalbrigt

Signed-off-by: Peter Lemenkov <lemenkov@redhat.com>