Do not rely on an specific memcached version for autodiscover test

[exekias]

Issue seen in travis:

======================================================================
ERROR: Test docker autodiscover starts modules from templates
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/go/src/github.com/elastic/beats/metricbeat/tests/system/test_autodiscover.py", line 40, in test_docker
    docker_client.images.pull('memcached:1.5.3')
  File "/usr/local/lib/python2.7/dist-packages/docker/models/images.py", line 415, in pull
    repository, tag, '@' if tag.startswith('sha256:') else ':'
  File "/usr/local/lib/python2.7/dist-packages/docker/models/images.py", line 295, in get
    return self.prepare_model(self.client.api.inspect_image(name))
  File "/usr/local/lib/python2.7/dist-packages/docker/utils/decorators.py", line 19, in wrapped
    return f(self, resource_id, *args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/docker/api/image.py", line 245, in inspect_image
    self._get(self._url("/images/{0}/json", image)), True
  File "/usr/local/lib/python2.7/dist-packages/docker/api/client.py", line 231, in _result
    self._raise_for_status(response)
  File "/usr/local/lib/python2.7/dist-packages/docker/api/client.py", line 227, in _raise_for_status
    raise create_api_error_from_http_exception(e)
  File "/usr/local/lib/python2.7/dist-packages/docker/errors.py", line 31, in create_api_error_from_http_exception
    raise cls(e, response=response, explanation=explanation)
ImageNotFound: 404 Client Error: Not Found ("no such image: memcached:1.5.3: No such image: memcached:1.5.3")

Old versions are sometimes removed, as this test is not about the module
itself we can use the latest tag here.

[ruflin]

I'm torn here. In general I prefer that we use fixed versions for consistency but it seem for the autodiscovery test the content does not really matter.