Readonly mounts - fixes 93

Fixes #93.

Depends on #598; tests should pass once #602 is merged and this branch is merged forward.
What exceptions are we expecting here? I don't think `Popen.wait` raises `CalledProcessError` and if the sub command fails we'd get a non-zero exit status.

I suppose this would catch things like https://github.com/ClusterHQ/flocker/issues/408#issue-39300979