Unify the query fields for the storage framework

This patch unifies the query fields in the storage framework for all
types. Note that the information is still computed on-demand, so if e.g.
the used disk space is not requested for the ‘file’ type, it won't be
computed on nodes.

Summary of changes:
- improve the LVM storage type to support multiple lvm fields in the
  LIST_FIELDS declaration and constant (not-computed via lvm commands)
  fields
- rename utils.GetFilesystemFreeSpace to utils.GetFilesystemStats
  returning tuple of (total, free)
- add used and free as valid fields for lvm-vg (use being computed as
  vg_size-vg_free)
- make allocatable accepted for all types (ones which are always
  allocatable always return True)
- add a new list field ‘type’ that gives the current selected type; not
  much useful today (except for understanding what the default output
  is) but in the future might help if we want to list multiple types
- add type, size and allocatable to the default output field list
- update the man page with details on how, for file storage, size ≠ used
  + free for non-mountpoint cases

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>