Initialize `mountpoint` and `check` fields. In case of an error
in `open` not having `mountpoint` or `check` initialized will
cause another exception. So this is mostly important in case of
error, but it is the right thing anyway.
Create a new OSTree deployment mount service that will set up bind
mounts inside the tree very much as it is done by OSTree in early
boot. This allows any stage to transparently work with OSTree
deployments.
Define the mount schema in the actual mounts at a higher level. This
is in preparation to give the modules more control over the `source`
and `target` properties.
