Glance image-list failed when image number exceed DEFAULT_PAGE_SIZE

The root cause of the problem is that DB2 does not allow
an empty string ('') for datetime field. So we set the
default value of datetime as None.

Fixes bug 1245947

Change-Id: Ie372efe70a068024298fbde4574e33e565cc6c9a
(cherry picked from commit 3f533a28fa79e5ad7d2703123565f34839cd9ff3)
Merge "Glance image-list failed when image number exceed DEFAULT_PAGE_SIZE" into stable/havana