Allow configuring alertmanager instance to fire against

Until Observatorium ships alertmanager functionality itself, we need to be able to fire alerts against an arbitrary alertmanager cluster. It would be sufficient to have one configuration for every tenant.

This is especially critical for ACM's tech preview. They want to integrate into the Alertmanager that ships with OpenShift Monitoring.

DoD:

• Admin can configure alertmanager endpoints to fire alerts against via a flag and or configuration file.