update_snapshot_id_in_launch_conf.py should accept multiple device_name and stack_prefix

Currently, the script used to update the snapshot id of a launch configuration: 
"files/aws-tools/update_snapshot_id_in_launch_conf.py" only accepts one pair of stack_prefix (--stack-prefix) and device (--device).

Script should be modified to accept multiple pairs.
Possibly using a hash/array of values as a parameter instead.