Add python-iso8601 to nova and glance dependencies

Nova and glance will shortly require the python-iso8601 package in
order to support non-UTC timestamps in the changes-since filter.

In particular, the following nova patch is blocked by devstack
failing due to the missing dependency:

  https://review.openstack.org/#change,4211

Change-Id: Iaae8812cf19c9ae1ad237b7fda7d99827aca1485
diff --git a/AUTHORS b/AUTHORS
index dc12105..b147827 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -7,6 +7,7 @@
 Dean Troyer <dtroyer@gmail.com>
 Devin Carlen <devin.carlen@gmail.com>
 Eddie Hebert <edhebert@gmail.com>
+Eoghan Glynn <eglynn@redhat.com>
 Jake Dahn <admin@jakedahn.com>
 James E. Blair <james.blair@rackspace.com>
 Jason Cannavale <jason.cannavale@rackspace.com>