Add message rate metrics to RabbitMQ check.

This is useful when you have a queue that's consumed very quickly. In
this case the Agent will always/nearly always report 0 for queue sizes.

New metrics:
    - `rabbitmq.queue.messages.rate`
    - `rabbitmq.queue.messages_ready.rate`
    - `rabbitmq.queue.messages_unacknowledged.rate`
More rabbitmq metrics
Fix a typo in the rabbitmq metrics.
Remove max_containers limit for Docker check
Cleanup Docker example config file
Add socket_timeout parameter to Docker check
Make the container size collection configurable
Remove warning when no containers are running
Make event creation in Docker check optional
Revamp Docker events: aggregate, put details in the body, add container name
Tag docker.containers.running by image
Drop docker.total_* metrics because it is not used and meaningless
Better code structure and comments for Docker check
Add a flag to exclude process args
Minor cleanup and fix
Make dogstatsd support magic tags

Tags host and device are converted into metric attributes
Make dogstatsd test fully deterministic
Make Docker service check monitor the container API only
Add config flag normalization for exclude_process_args
Allow empty host in dogstatsd to unset the attribute