`st2ctl status` should report nginx, MongoDB, PostgreSQL & RabbitMQ status

st2ctl status reports on the status of st2 processes. It does not report anything about nginx, MongoDB, PostgreSQL or RabbitMQ.

This can be confusing for users who are experiencing issues with those services. They run st2ctl status, and it appears to report that everything is working, when in reality RabbitMQ is broken because they ran out of disk space. This wastes everyone's time.

The challenge here is that those dependencies may be running on a separate system.

Perhaps we should add a check that makes a test connection to those services (based on their definition in /etc/st2/st2.conf, and reports the results?