state: change block-device representation in state

Also, start on datastores in state. Can't do much
more there without charm metadata.
Update worker/diskformatter

Various updates, revolving around disks being
represented as first-class entities in state.
worker/diskformatter: fix tests
apiserver/diskmanager: update to state changes
worker/diskmanager: fix import grouping
cmd/jujud: fix DiskManager test