FileSystem: Add FileSystem._prepare()

Add a new method to FileSystem class : _prepare()
This methods is a generic method to factorize all similar code for
filesystem actions like start, format, mount, ...
It applies an action on a component list, taking care of dependencies.
It also adds call to module load or unload if requested.
Ticket #82: Return an error if command failed and comp is offline

When an action failed but the component state was OFFLINE, Shine
failed to return an non-zero error code.
It now returns TARGET_ERROR which is appropriately converted where
needed to an non-zero error code.
This fixes Format, Fsck, Tunefs and Execute.
Ticket #152: umount/mount client on update when needed

If mount_path or mount_options are modified, all clients are now
unmounted and then mounted instead of just printing to do it by hand,
AFTER the config files have been updated, which is too late.