Handle TZ change in iso8601 >=0.1.12

The iso8601 lib introduced a change such that if running on python
3.2 or later it internally uses the python timezone information
instead of its own implementation. This does not change direct
date handling, but when converting this value there is a slight
difference where now python 2.x will show UTC times as "UTC",
but on python 3 they will end up with "UTC+00:00".

The to_primitive call for DateTime fields was doing an exact match
on "UTC" to determine whether to include "Z" in the resulting string.
This updates that handling to recognize either of the new values

Closes-bug: #1744160

Change-Id: I505434facc7adc4a479f67eeedb31cf7b4bf7caf

keystone/common/utils.py

@@ -498,7 +498,8 @@ def isotime(at=None, subsecond=False):
                      if not subsecond
                      else _ISO8601_TIME_FORMAT_SUBSECOND)
     tz = at.tzinfo.tzname(None) if at.tzinfo else 'UTC'
-    st += ('Z' if tz == 'UTC' else tz)
+    # Need to handle either iso8601 or python UTC format
+    st += ('Z' if tz in ['UTC', 'UTC+00:00'] else tz)
     return st