Fix: `alertmanager.yml` should have valid `receiver` field.
Fix: `alertmanager.yml` should have valid `receiver` field.

@see https://github.com/prometheus/alertmanager/issues/234

@see-also README in https://github.com/prometheus/alertmanager/blob/master/README.md :
  # The root route must not have any matchers as it is the entry point for
  # all alerts. It needs to have a receiver configured so alerts that do not
  # match any of the sub-routes are sent to someone.

William-Yeh/ansible-prometheus