Alertmanager 0.13.0 doesn't support single dash long options, so '-config.file'
for example is parsed as '-c', which leads to the service not starting.
The reason being less mental overhead when reading upstream
documentation. Examples can be pasted right into the configuration
instead of translating to Nix attrset first.
The current default value of listenAddress = null blows up:
$ nixos-rebuild build
error: cannot coerce null to a string, at
.../nixpkgs/nixos/modules/services/monitoring/prometheus/alertmanager.nix:97:16
With listenAddress = "" we use the same default as upstream and there is
no blow up :-)
