Merge branch 'master' into 0.7-staging
Clean up better from previous unit-test runs

This makes sure we unmount existing mounts (as well as removing the
devmapper devices), and it fails with proper logs rather than just
panic()ing.
Migrate AUFS containers to devmapper
Merge branch 'migrate-aufs-containers' into v0.7-rc3

Conflicts:
	runtime.go