Adding an experimental multicluster ingress API

[nikhiljindal]

First stab at defining a MultiClusterIngress API.

There are 2 high level resources here: MultiClusterIngress and MultiClusterService.

Key differences with the current CLI model:

• MultiClusterService can be used to create the required service in appropriate multiple clusters (based on a cluster selector). With the existing CLI model, users have to create the required service in all clusters.
• MultiClusterIngress does not create Ingress resource in target clusters. Only services are created in target clusters (via MultiClusterService). This is different than CLI which creates Ingress resource in all target clusters.

Advantages:

• Users dont have to create Services in target clusters. This can now be done automatically.
• Since cluster selection can now be done with service instead of ingress, different paths on the same MultiClusterIngress can route traffic to different set of multiple clusters. For example, /foo can route traffic to clusters A and B and /bar can route traffic to clusters B and C for a single MultiClusterIngress.
• Be able to front Services with MultiClusterIngress without having to expose the Service directly with an external public IP or DNS record.

This is still Work in Progress (WIP). Sharing the API definition early to get reviews. Please review and add comments.

[nikhiljindal]

cc @csbell @bowei @rramkumar1 @agau4779 @thockin @G-Harmon

[k8s-ci-robot]

@nikhiljindal: The following test failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
pull-kubernetes-multicluster-ingress-test	1c931daea39cbb6e8d185b8f22f986b7a92ba057	link	/test pull-kubernetes-multicluster-ingress-test

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

Instructions for interacting with me using PR comments are available [here](https://git.k8s.io/community/contributors/guide/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes/test-infra](https://github.com/kubernetes/test-infra/issues/new?title=Prow%20issue:) repository. I understand the commands that are listed [here](https://go.k8s.io/bot-commands).