Enable users to create HTTPS ingresses

[nikhiljindal]

For now, we support creating http ingresses only.
Need to add SSLCerts management code to be able to create HTTPS ingresses

[nikhiljindal]

This includes 2 parts:

• frontend HTTPS (from client to GCLB): https://github.com/kubernetes/ingress-gce#frontend-https
• backend HTTPS (from GCLB to the kube service): https://github.com/kubernetes/ingress-gce#backend-https

cc @csbell @G-Harmon

[nikhiljindal]

With #92 and #97, we now support frontend https with SSL certs stored as secrets.

Next steps are to support:

• frontend HTTPS with certs using the pre-shared-certs annotation.
• backend HTTPS

[nikhiljindal]

We now support frontend HTTPS with pre-shared certs and backend HTTPS.
We need to add support for https only.

#131 tracks adding an e2e test.

[nikhiljindal]

We now have e2e tests (https://k8s-testgrid.appspot.com/sig-multicluster-kubemci#kubemci-ingress-conformance) and documentation: #143