Configure RabbitMQ user tags in nova-cell role

The RabbitMQ 'openstack' user has the 'administrator' tag assigned via
the RabbitMQ definitions.json file.

Since the Train release, the nova-cell role also configures the RabbitMQ
user, but omits the tag. This causes the tag to be removed from the
user, which prevents it from accessing the management UI and API.

This change adds support for configuring user tags to the
service-rabbitmq role, and sets the administrator tag by default.

Change-Id: I7a5d6fe324dd133e0929804d431583e5b5c1853d
Closes-Bug: #1875786