Use timeutils.isotime() in images view builder

Rather than using a direct call to strftime() with the same format
as timeutils.isotime() just use isotime() itself.

Full context here:

  http://lists.openstack.org/pipermail/openstack-dev/2014-April/033971.html

Change-Id: Ifeb9e24566e220f80cf8051ec16b8c868908c642
Merge "Use timeutils.isotime() in images view builder"