Enable/Fix rancher-metadata backend config method

[gaieges]

It's my understanding that @SantoDE is working on something to enable the ability to have traefik pull from rancher-metadata in the local environment (the fix for the method to pull from the API is in another ticket) as a backend for configuring rules.

This ticket is to track that as I didn't see another ticket open for it. Feel free to close if there is.

What version of Traefik are you using (traefik version)?

Version:      v1.2.1
Codename:     morbier
Go version:   go1.7.5
Built:        2017-03-27_03:19:40PM
OS/Arch:      linux/amd64

What is your environment & configuration (arguments, toml...)?

docker-compose handles all this and rancher cli deploys it:

version: '2'
services:
  traefik:
    image: traefik:1.2-alpine
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock
    ports:
      - '443:443'
      - '80:80'
      - '8000:8000'
    command: 
      - --loglevel=debug
      - --rancher
    labels:
      traefik.enabled: 'false'
      io.rancher.container.pull_image: always
      io.rancher.container.create_agent: 'true'
      io.rancher.container.agent.role: environment
      io.rancher.container.global: 'true'

What did you do?

Spun up stack in rancher via cli.

What did you expect to see?

Expected it to see all of the local environment instances that have traefik.port or traefik.frontend.rule labels to be present in the traefik web ui

What did you see instead?

Exception in traefik logs:

level=error msg="Cannot get Rancher Environments Unknown schema type [environment]\ngithub.com/containous/traefik/vendor/github.com/rancher/go-rancher/client.(*RancherBaseClientImpl).doList\n\t/go/src/github.com/containous/traefik/vendor/github.com/rancher/go-rancher/client/common.go:299\ngithub.com/containous/traefik/vendor/github.com/rancher/go-rancher/client.(*EnvironmentClient).List\n\t/go/src/github.com/containous/traefik/vendor/github.com/rancher/go-rancher/client/generated_environment.go:119\ngithub.com/containous/traefik/provider.listRancherEnvironments\n\t/go/src/github.com/containous/traefik/provider/rancher.go:293\ngithub.com/containous/traefik/provider.(*Rancher).Provide.func1.1.1\n\t/go/src/github.com/containous/traefik/provider/rancher.go:253\ngithub.com/containous/traefik/safe.(*Pool).Go.func1\n\t/go/src/github.com/containous/traefik/safe/routine.go:77\ngithub.com/containous/traefik/safe.GoWithRecover.func1\n\t/go/src/github.com/containous/traefik/safe/routine.go:141\nruntime.goexit\n\t/usr/local/go/src/runtime/asm_amd64.s:2086"