UTC ISO8601 from timestamp

Taken from glance xattr.py module. This function converts timestamp
formated dates into a UTC Iso 8601 date. It is being required in
multiple places now (mostly on clients printing dates) in order to make
dates human readable.

Change-Id: I6f19325a4c2df241a0ef76bd256280a2930d9265
Merge "UTC ISO8601 from timestamp"