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
Merge "Glance image-list failed when image number exceed DEFAULT_PAGE_SIZE"