URI stringification constructs malformed URIs.

Setting docker_registry="https://docker-cache.example.com/ and then pulling an image named {{org/image-name:latest, }}the Docker image puller ends up constructing a malformed URL for the manifest:

Pulling image 'org/siri-centos6:stage' from 'docker-manifest://docker-cache.example.com:443org/image-name?latest#https' to '/tmp/mesos/store/docker/staging/LGArHA'