volumes: absolute path becomes relative if it is in an environment variable

[stefanfoulis]

This used to work with fig, but no longer seems to work in docker-compose==1.2.0rc3:

myservice:
  volumes:
    - ${DOCKER_CERT_PATH}:/certs

DOCKER_CERT_PATH contains an absolute path to a directory (e.g /Users/me/.boot2docker/certs/boot2docker-vm).
Instead of mounting that, it will make it relative to the directory of the docker-compose.yml file.

Probably the code checks somewhere if it starts with / before expanding the variable, since setting it to - /${DOCKER_CERT_PATH}:/certs works.

[aanand]

Hmm, we should probably move the expanduser and expandvars logic from service.py to config.py.