Ability to Configure Typha for Calico CNI

[gjtempleton]

1. Describe IN DETAIL the feature/behavior/change you would like to see.
The Calico deployment in k8s 1.12 currently sets the typha_replicas to 0 and typha_service_name: "none" in https://github.com/kubernetes/kops/blob/master/upup/models/cloudup/resources/addons/networking.projectcalico.org/k8s-1.12.yaml.template

The recommendation from the Calico docs when using the k8s APIServer as the datastore as kops does is that once a cluster goes above 50 nodes then Typha is used to lessen the impact on the k8s API Servers from Felix: https://docs.projectcalico.org/v3.4/getting-started/kubernetes/installation/calico#installing-with-the-kubernetes-api-datastoremore-than-50-nodes

I've seen #7051 and this doesn't change this behaviour and the recommendation is still the same with Calico v3.7 : https://docs.projectcalico.org/v3.7/getting-started/kubernetes/installation/calico#installing-with-the-kubernetes-api-datastoremore-than-50-nodes

To allow users to conform with this recommendation I propose adding the ability for users to modify their calico config to make use of Typha via editing their cluster and adding a typhaReplicas option to their networking: calico: ... stanza.

e.g.

...
networking:
  calico:
    crossSubnet: true
    typhaReplicas: 3
...

Setting typhaReplicas to a non zero positive integer would then have a number of effects:

• Create a service for calico-typha
• Set the typha_service_name to calico-typha in the Calico configmap to configure the calico-node pods to connect to Typha rather than directly to the API Server.
• Set the number of replicas for the calico-typha deployment to the same value as typhaReplicas

The intention of this would be to act as an intermediate step between the complexity allowed for by: #6872 where people can scale their clusters to a massive degree using BGP Route Reflectors and the current 0 Typha, API server datastore default setup.
2. Feel free to provide a design supporting your feature request.

[gjtempleton]

As discussed at office hours today I'll start work on a first cut of this over the next few days.