add reverse channel to the user to show egress IPs

[szuecs]

Users want to know which IPs they will have for static egress IPs.
A ConfigMap has no status field, so we have to use something like a label or annotation to provide this information back to the user.

A user can get the egress IP with:

% kubectl get configmap --show-labels -l egress=static
NAME        DATA      AGE       LABELS
egress-t1   1         1m        egress=static,status=<ip1>|<ip2>|<ip3>

[aermakov-zalando]

Another option would be to create a CRD for egress definitions.