Use full-stripe size in LVM growth

LVM has issues when growing stripped volumes, so it's best to specify
the growth in exact multiples of the full stripe size (as precise as
possible). For this we need to do a couple of changes:
  - in LVM Attach(), we query additionally the VG extent size and the LV
    stripe count; since this makes lvs return a (possibly) multi-line
    output, we now split it into lines and only take the last one
  - in LVM Grow(), we round up the increase in multiples of the full
    stripe size

The patch also sets the correct target size in DRBD growth.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Olivier Tharan <olive@google.com>