Make max-online-resize an option.

In fb246a02eb2ed330d3cc37f5795b3ed026aabe07 we introduced an
ext4 option to allow root filesystems to be resized up to 1PB.
This appears to cause an ext4 resize2fs bug in some images.

When the issue occurs an image will hit a kernel bug when
cloud-init runs the resize2fs command during first boot:

  kernel BUG at fs/ext4/resize.c:409!

In this commit we add a new option for max-online-resize
which can be used if a really large root partition is
desirable.

The root cause of all this is really a design problem in
DIB/TripleO at the moment in that we shouldn't have to worry about the
max size of the root file system when creating our images.  Ideally we'd
just mkfs on the root file system itself. Much more efficient,
avoids this problem altogether...

I think the best thing to do today to avoid this is make setting
max-online-resize an option in DIB. This will allow us to stick
to the (well tested) ext4 defaults for most cases, and if someone has
need for a large root filesystem they can easily bump the setting. This
may be temporary until we either fix the design... or the ext4 fix is
released.

Change-Id: I371f62555d2753cec48790c8fd811c4342af925c
Closes-bug: #1280709