Add disk checks

[stevendborrelli]

Add the following checks:
root disk:
/ 95% full disk, 95% full on inodes

As part of the docker check package:
/var/lib/docker: 95% full disk, 95% full on inodes

[langston-barrett]

/ 90% full disk is already done

[langston-barrett]

Here's the kicker - I can't find any way to get the inode usage by path. Inode usage really only makes sense when we're talking about filesystems, but we don't necessarily know which filesystem / and /var/lib/docker will be mounted on.