Add command to validate state of deployment

[dpoirier]

It's be very helpful if there was a command that would check what it could to see if things had deployed right and setup had been done. A partial list:

• Security groups exist and have the right authorizations
• Load balancer set up right
• Autoscaling group set up right
• Expected servers exist
• Passwords are installed right
• Instances are the expected sizes
• DB slave has been set up to replicate
• DB master & slave work
• Worker is running celery & celerybeat
• Cache is running redis, rabbitmq, etc.
• Web servers are running gunicorn, nginx, etc
• Nginx is not rejecting outright http requests that use any of the configured Host names.
• healthcheck.html works on each web server

The output when problems are detected should, where possible, give hints for correcting (e.g. "run reset_slaves to set up DB slave replication"), but the command should not fix anything itself; we always want it to be safe to run it.

I was thinking a command like fab staging doctor.