Return reference to object providing Endpoint

[antoineco]

Summary

This adds a new field Target to the ingress.Endpoint type, which references the object providing that Endpoint.

Use case

I am writing a controller for Apache 2 and want to provide the ability to enable stickiness in front of Tomcat backends. The standard way to achieve this is to set the route= parameter for every BalancerMember (equivalent of upstream.server in nginx). This parameter must match the value of jvmRoute= on the backend.

Without a reference to the Pod providing the endpoint I can't reliably set a route Apache 2 and Tomcat can agree on, like the Pod's name.

ref: https://httpd.apache.org/docs/2.4/mod/mod_proxy_balancer.html#stickyness_implementation

[k8s-reviewable]

This change is 

[coveralls]

Coverage decreased (-0.01%) to 44.105% when pulling 6035e55 on antoineco:ep-targetref into 65e8cec on kubernetes:master.

[aledbf]

/lgtm

[aledbf]

@antoineco thanks!