Convert volume_list rpc to new style result

This is a big change, because we need to cleanup its users too.

The call and thus LUVerifyDisks LU used to differentiate between failure
at node level and failure at LV level, by returning different types in
the RPC result. This is way too complicated for our needs.

The patch changes to new style result (easy change), and then:
  - changes LUVerifyDisks.Exec() to return a tuple of 3-elements
    instead of 4-elements; we collapse the «nodes not reachable» and
    «nodes with LVM errors» in a single dict
  - changes gnt-cluster to parse 3-element results and simplifies the
    different by-error handling code

Note that the status is added in ganeti-noded, and not in the function
itself, as the function is used in other places too.

This was tested with down nodes and broken VGs.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>