Remove infinite loop in delete snapshot, fail fast, update stdout

deleting snapshots could queue the same snapshot multiple times
if it took any amount of time to complete. This could 
overload the SDC API.

rename MAX_SNAPSHOTS to SNAPSHOTS_TO_KEEP
rename MIN_FREE to MIN_FREE_MB
use HOME instead of ~
really really fail fast if any command fails
Run every hour instead of every 15 minutes