Instead of only testing single instance Alertmanagers, this patch
enables individual tests to spin up Alertmanager clusters.
In addition it adds two tests:
1. A test firing alerts against a cluster, expecting to only receive a a
notification by one of the Alertmanager instances in the cluster.
2. A test firing alerts both against a single instance as well as a
cluster, making sure the output equals.
Signed-off-by: Max Leonard Inden <IndenML@gmail.com>
Max Leonard Inden