ingress: support one ingress and use hostnames/SNI (allows better letsencrypt support)

[Cryptophobia]

From @tmc on June 12, 2017 2:50

Right now the experimental native ingress creates an ingress per app, this typically implies manual steps, dns records, etc -- lowering the utility and ease.

If native ingress support allowed the utilization of a single ingress in SNI mode converting a deis installation to use letsencrypt via kube-lego would be a single annotation and dns record configuration.

Refs https://github.com/deis/workflow/issues/708

Copied from original issue: deis/workflow#824