Allow units in ipolicy disk/mem CLI changes

This makes "gnt-cluster modify --specs-mem-size max=16g" work. The
downside (due to how we iterate) is that error messages are not very
clear:

  $ gnt-cluster modify --specs-mem-size max=16a
  Failure: prerequisites not met for this operation:
  error type: wrong_input, error details:
  Invalid disk ({}) or memory ({'max': '16a'}) size in policy: Unknown unit: a

Also updates the help of these commands, which was very confusing
before (at least for me).

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