Add Capistrano task to remove application from the load balancer

[tpendragon]

Acceptance Criteria

• Running cap --hosts=<hosthere> staging application:remove_from_nginx will prevent a node from serving traffic on the load balancer
• Running cap --hosts=<hosthere> staging application:serve_from_nginx will allow a node to serve traffic

Technical Implementation

Add a health monitor that looks at a file on disk and reports a failure if it exists, so that nginx will fail its health check and pull it out.

Example PR: https://github.com/pulibrary/pdc_describe/pull/1985/files

We're really hoping https://github.com/lbeder/health-monitor-rails/pull/127 will get merged.

First Step

Update the health-monitor-rails gem so it has the new check.