Be smarter about stashing changes for validation

Don't save empty parts of the model, that way they won't be
reused if we repopulate fields. This still allows for flagging
fields as long as we don't use reuse_old, but lets us swap the
page without flagging untouched fields if necessary

Change-Id: I344ffba11cc27c5e325e91bc50727d0a7e734732
Signed-off-by: Ryan Barry <rbarry@redhat.com>