Use amtool in the alertmanager pod instead of silencing job

[haasad]

Reduces the number of manual steps needed (no cleanup of jobs etc.) and reduces the wall of code in the how-to

[simu]

Might make sense to update the partials to always use amtool instead of just dropping the partial here. That way, we'd improve all the how-tos that create silences instead of just one of them.

[haasad]

I created partials for creating and expiring silences with amtool. I don't have the capacity at the moment to migrate all existing silence jobs to the new method. But like this we can reuse the new method when needed.