Update alerta_prometheus.py to honor the silence from ack setting

[dakotacody]

1. ALERTMANAGER_SILENCE_FROM_ACK variable was not being honored to actually disable the silence from an ack.
2. Set the silence seconds if ALERTMANAGER_SILENCE_DAYS variable is not set.
3. If a silence is found for an alert that is getting a status change of open or closed we should probably remove the silence in alertmanager if it exists.