Include timezone in metadata validUntil value and use UTC time
The python datetime module doesn't append the timezone in its
isoformat() output, so add a Z indicating that the time is
UTC time. Also generate the output using utcnow() rather than
now() so the times line up.
https://fedorahosted.org/ipsilon/ticket/137
Signed-off-by: Rob Crittenden <rcritten@redhat.com>
Reviewed-by: Patrick Uiterwijk <puiterwijk@redhat.com>