Allow stripping of path-prefix in a portable way

[micw]

I'd like to strip paths for forwarded URIs in a way that is compatible with other ingress controllers.

Example:

apiVersion: extensions/v1beta1
kind: Ingress
metadata:
  annotations:
    nginx.ingress.kubernetes.io/rewrite-target: /$1
  name: test
spec:
  rules:
  - host: test.example.com
    http:
      paths:
      - backend:
          serviceName: test
          servicePort: http
        path: /something/(.*)

This maps /something to / on the backend service. It works pretty well with nginx but the regular expression on the path breaks compatibility with other ingress controllers (e.g. traefik 1+2).
The expression part is required because it generates the following config:

rewrite "(?i)/something/(.*)" /$1 break;

Without the expression part, $1 is not available for rewrite.

Possible solutions:
Variant1: always add (.?) at the end of the 1st rewite parameter
Variant1a: add an annotation to enable Variant1
Variant1b: add (.?) at the end of the 1st rewite parameter if the 2nd parameter uses $1 but there's no capture group on the 1st parameter
Variant2: add an annotation that allows to override the 1st rewite parameter

Personally I see Variant1b as the best solution. It does not break any existing deployments and adds the requested feature without any additional annotations.

/kind feature

[aledbf]

@micw unfortunately, this is something we already tried in the past for the path field.
Please check #555 and #4989.

For that reason, I am closing this issue.

[micw]

@aledbf It seems that #555 was closed after the discussion lost focus. My proposal above would be an easy way to solve 90% of the portability issues with a minimal code change. Would you be open to talk about it?