Correct App-Root kubernetes behavior

[dtomcej]

What does this PR do?

Corrects App-Root kubernetes Behavior

Motivation

Fixes #3589

More

• Added/updated tests
• Added/updated documentation

Additional Notes

We will need to update documentation to reflect being unable to mix regex with app-root.

The ingress:

apiVersion: extensions/v1beta1
kind: Ingress
metadata:
  name: whoami
  annotations:
     traefik.ingress.kubernetes.io/app-root: /app1/index.html
spec:
  rules:
  - host: localhost
    http:
      paths:
      - path: /
        backend:
          serviceName: whoami
          servicePort: http

Now results in:

$ curl -v localhost
* Rebuilt URL to: localhost/
*   Trying ::1...
* TCP_NODELAY set
* Connected to localhost (::1) port 80 (#0)
> GET / HTTP/1.1
> Host: localhost
> User-Agent: curl/7.54.0
> Accept: */*
> 
< HTTP/1.1 302 Found
< Location: http://localhost/app1/index.html
< Date: Tue, 10 Jul 2018 19:31:55 GMT
< Content-Length: 5
< Content-Type: text/plain; charset=utf-8
< 
* Connection #0 to host localhost left intact

[dtomcej]

Should also note that the traefik.ingress.kubernetes.io/redirect-permanent annotation can also determine if a 301 or a 302 should be sent.

[ldez]

LGTM

[dtomcej]

Documentation added. Ready for review + merge

[nmengin]

LGTM