Configure NRPE to check /dev/mapper/backup-data

This pull request installs and configures NRPE which is then used to check for disk space warnings on `/dev/mapper/backup-data/`, where data is backed up to. Warnings are created a 20% free disk, and criticals are raised at 10% free disk. It opens tcp/5666 locally. Once merged, tcp/5666 needs opening on the vSE.