Provision RabbitMQ configuration

Summary:
RabbitMQ clusters can be configured in pillar/saas/rabbitmq.sls

HTTP API requests through the management plugin are fired
to ensure each cluster is configured as expected.

Vault configuration
The salt-primary node policy has read access to the credentials defined
in rabbitmq_clusters pillar, so it can run the saas-rabbitmq role.

Policies are now templates, and the node policy reads the rendered template
version from /srv/policies instead of Salt roles/ folder.

Ref T752.

Test Plan: Provision white-rabbit cluster

Reviewers: DorianWinty

Reviewed By: DorianWinty

Maniphest Tasks: T752

Differential Revision: https://devcentral.nasqueron.org/D2793