Fix Rewrite-target regex

[dtomcej]

What does this PR do?

Removes an added slash in the Redirect-target annotation regex

Motivation

Fixes #3698, slash was accidentally added in #3582

More

• Added/updated tests
• Added/updated documentation - None needed, bugfix

Additional Notes

The ingress:

apiVersion: extensions/v1beta1
kind: Ingress
metadata:
  name: whoami
  annotations:
     traefik.ingress.kubernetes.io/rewrite-target: /demo
spec:
  rules:
  - host: localhost
    http:
      paths:
      - path: /testing
        backend:
          serviceName: whoami
          servicePort: http

Now properly creates the following path rewrites:

$ curl -v localhost:31117/testing -silent 2>&1 | grep GET

> GET /testing HTTP/1.1
GET /demo HTTP/1.1

$ curl -v localhost:31117/testing/ -silent 2>&1 | grep GET

> GET /testing/ HTTP/1.1
GET /demo/ HTTP/1.1

$ curl -v localhost:31117/testing/bacon -silent 2>&1 | grep GET

> GET /testing/bacon HTTP/1.1
GET /demo/bacon HTTP/1.1

[nmengin]

LGTM 👍

[mmatur]

LGTM