turku should write metadata information for each storage directory

Hi,

On the turku-storage units, it's hard to tell what host / Juju environment / Juju service / Juju unit a particular storage directory is on disk. It would be nice to have metadata written out, something like a ".turku-storage-metadata.txt" with the source name, machine, Juju environment, Juju service name, Juju unit name, etc.