Correct spec filenaming for disk_manager and disk message

Signed-off-by: Rob Day-Reynolds <rdayreynolds@pivotallabs.com>
Move DiskUtil into own file and out of Message namespace

Signed-off-by: Rob Day-Reynolds <rdayreynolds@pivotallabs.com>
Separate disk messages into individual files

Signed-off-by: Rob Day-Reynolds <rdayreynolds@pivotallabs.com>
Start to backfill tests for MountDisk

Signed-off-by: Rob Day-Reynolds <rdayreynolds@pivotallabs.com>