Use the prometheus-alertmanager package from stretch-backports

The version in stretch-backports has modern clustering support, but
unfortunately drops the UI due to it being impossible to build in
Debian (https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=894131).

We upgrade the alertmanager package first, and will later find a
solution for the UI. It's possible to manage the alertmanager using
'amtool' in the meantime.