Add the "storage" prefix to all Ceph-based commands in both the CLI and
the API. This partially abstracts the storage subsystem from the Ceph
tool specifically, should future storage subsystems be added or changed.
The name "ceph" is still used due to the non-abstracted components of
the Ceph management, e.g. referencing Ceph-specific concepts like OSDs
or pools.
Previous to this, if once force-migrated a VM, the previous_node value
would be updated to the current node, which is likely never what an
administrator would want. Change this functionality so that the previous
node value is not changed, and update the documentation to reflect this.
Joshua Boniface