Test date with UTC instead of local timezone.

iso_format test uses local timezone so it can fail at certain times
of the day depending on which timezone the tests are run in.

Fixes bug 1139131

Change-Id: I8fe423d96c438b46a9307fe1ecf648707183ff1c

glance/tests/functional/test_bin_glance_cache_manage.py

@@ -85,7 +85,7 @@ def iso_date(self, image_id):
 
         exitcode, out, err = execute(cmd)
 
-        return datetime.date.today().isoformat() in out
+        return datetime.datetime.utcnow().strftime("%Y-%m-%d") in out
 
     def test_no_cache_enabled(self):
         """