Upstream has apparently changed the configuration format and is now
throwing an error when the `encrypt_sse` option is set. According to the
current version of the documentation encryption moved to the
`sse_config` option that (is optional and) offers all the features we do
not use or care about for this test.
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.
