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: Ie35ef49546dd322c2630c77d528a197ff295c3ae
Closes-bug: #1715486

openstack/ironic,openstack/ironic