Migrate check_disk_5_2 to root_disk_space for labs.

Replaces the NRPE check_disk_5_2 check with the root_disk_space check.

Enables usage of change If6eedb0b on the server side, prior to change I8cdee520 removing the check_disk_5_2 check.

Change-Id: If5b834f83329dc114fa0cc4176226f3ec22f5942