Allow mount services to return None, which means they have not
actually mounted anything within the mount root. This might be
because they have bind mounted directories within the tree.
These mounts do not need any path translation.
The previous commit gave the individual mounts more control over the
source and target properties. Do not require them at the global
schema but hand the control if they are optional over to the modules.
Introduce a new specialized service manager class `MountManager` to
manage mounts. It uses the newly introduced `DeviceManager` to look
up devices and stores the reference to the mount point root path.
See the commit that introduced the `DeviceManager` for more info.
Allows stages to access file systems provided by devices.
This makes mount handling transparent to the stages, i.e.
the individual stages do not need any code for different
file system types and the underlying devices.
