alexanderadam

* Shows a message while asserting like: 

ok: [host] => {                  
    "msg": "disk usage 4.2B of total 20.0GB (21.0%) (should exceed limit 90.0%)"
}

* Note this only looks at first mount point on current node
* Fails if disk is near-full
* Last step pushes to a push-based monitoring service, which will alert us if it doesn't get there after some time
* Need to setup a  variable `disk_limit`, which is the max acceptable usage ratio, e.g. set it to 0.8 if you want to keep disks within 80% of max size