use openstack env vars in swift flavour config
Merge pull request #304 from ActiveState/swift-env

use openstack env vars in swift flavour config
store /images and /tags files

this is required for the static registry PR,
https://github.com/dotcloud/docker/pull/4607

with this change, 'docker pull' can be run against the underlying data storage
without involving the registry middleman. a simple redirecting http proxy is
still required to validate the endpoint.

for details, see https://github.com/ActiveState/static-docker-registry