Allow iallocator to work without LVM storage

Currently, the iallocator interface requires LVM storage, due to the
way it computes the node storage information.

By changing the code to understand that GetVGName() can return None,
and by setting the disk_total/disk_free node parameters to the value
zero, we can workaround this issue until proper multi-storage
reporting is implemented.

The hail iallocator plugin works without changes once this is
implemented, as it already handles non-LVM storage as "foreign"
storage (with unlimited capacity).

The patch also changes a bit how the parameters are retrieved and
used; hopefully this is a bit safer and more readable.

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