mgr/deepsea: use ceph_volume output in get_inventory()
DeepSea is being updated to use ceph_volume internally (see SUSE/DeepSea#1517 and jschmid1/DeepSea#6). Once this is done, the mgr_orch.get_inventory runner will just be returning the raw ceph_volume output, so this PR updates the DeepSea mgr module to match. There's also a couple of small cleanup commits. We probably don't want to merge this until the DS PR is in though.