Kubernetes: support Ingress hostname with wildcards/regexp

[hjacobs]

Hopefully the Kubernetes spec gets extended to define wildcard/regexp behavior: kubernetes/kubernetes#41881

[hjacobs]

@aryszka maybe you can describe how Skipper treats wildcards/regexp by default.

[schlomo]

I also need to create a wildcard subdomain like *.foo.bar.com.
FYI @mikkeloscar @tlossen

[szuecs]

@hjacobs I changed the title to match @schlomo's comment, because with #667 we support path-modes, which can be either Kubernetes API spec like, a Go regexp or PathSubtree match like in skipper. I prefer PathSubtree match, which does not lead to ambiguous routing trees and fits better the Kubernetes user documentation. We also switch our setup to PathSubtree in zalando-incubator/kubernetes-on-aws#1139 and dumped all ingresses to see if anyone is actual relying on regexps, and turns out no one is using regexps in our clusters.
Path handling is also documented in https://opensource.zalando.com/skipper/kubernetes/ingress-usage/#ingress-path-handling