nixos/prometheus: require one alertmanager configuration parameter

This commit adds an assertion that checks that either `configFile` or
`configuration` is configured for alertmanager. The alertmanager config
can not be an empty attributeset. The check executed with `amtool` fails
before the service even has the chance to start. We should probably not
allow a broken alertmanager configuration anyway.

This also introduces a test for alertmanager configuration that piggy
backs on the existing prometheus tests.