Alertmanager: Pass pod ip to --cluster.listen-address
Alertmanager: Pass pod ip to --cluster.listen-address

Alertmanager needs a routable IP address via the
`--cluster.listen-address` in order for other nodes in an Alertmanager
cluster to be able to reach it. This patch sets the
`--cluster.listen-address` to the k8s pod ip + `6783` port.

coreos/prometheus-operator,jescarri/prometheus-operator,jescarri/prometheus-operator,coreos/prometheus-operator