Using time units ['1s','1m','1h','1d','1w'] for silence in plugin 'prometheus'

[veter2005]

Prometheus plugin allows to use day unit for setting silence duration, but it's useful to use other units like minutes, hours, and so on.

Replace ALERTMANAGER_SILENCE_DAYS with ALERTMANAGER_SILENCE_PERIOD that can be one of the units ['1s','1m','1h','1d','1w']

[veter2005]

My version of how it can be done here
master...veter2005:alerta-contrib:issues_400

[hyberdk]

if it were me I would keep backwards compatibility, so that if you dont do any day unit, it assumes that its days..