Fix growing of DRBD disks

Patch 03941a26 made disk growth VG-aware, but the code has a bug: it
only works for LD_LV disks, and not LD_DRBD. To fix it, we add a
generic function to compute the per-VG growth requirements, and switch
to using that instead of manual build.

Furthermore, since the function _CheckNodesFreeDiskPerVG always takes
its input from this new function or _ComputeDiskSizePerVG, we change
the latter to always return a dict (possibly empty), instead of either
a dict or None. This makes the types more consistent.

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