s/alarm/alarm_id/ in alarm notification

During the review cycle for the alarm notifier, the notification payload
was changed from containing the entire alarm representation to just the
alarm UUID.

However, the field name remained as 'alarm' which could be confusing.

Hence, we change this to alarm_id.

Change-Id: I5d9409df92010c7c89ad1f108b074bbf8487bb90
Merge "s/alarm/alarm_id/ in alarm notification"