Use newer location for iso8601 UTC
Use newer location for iso8601 UTC

Newer versions have moved from iso8601.iso8601.Utc() to
just iso8601.UTC, causing tests to fail under py35.

Change-Id: I387c85f649316b26109eb1bc21da5ce24c450852
Closes-bug: #1715486

openstack/manila,bswartz/manila,bswartz/manila,openstack/manila