Value of hbase.master.snapshot.ttl is not used

When creating a snapshot, users can explicitly specify the TTL to be used. If no TTL is specified, then the SnapshotDescription is initially created with a TTL of -1 to indicate FOREVER.

When the SnapshotDescription runs through SnapshotDescriptionUtils#validate, the TTL is checked to see if the default value of hbase.master.snapshot.ttl should be applied. The value from the config is only applied if the TTL == 0, but it should be -1.

This has another nasty side-effect: any user who creates a snapshot and explicitly sets {TTL => 0} will find that their snapshot gets its TTL from hbase.master.snapshot.TTL.