add required python modules(six,iso8601,fixtures) to requirements.txt

[DennyZhang]

These modules of six, iso8601, fixtures, are requiementd by following tempest modules.
But they are missing in requirements.txt.

,-----------
| ./tempest/openstack/common/log.py:42:from six import moves
| ./tempest/openstack/common/timeutils.py:25:import iso8601
| ./tempest/api/compute/admin/test_aggregates.py:18:import fixtures
| ./tempest/test.py:22:import fixtures
`-----------

Due to network issue, run_test.sh fail for the first time. Then I try it agin, and found this problem.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/46665

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/46665
Committed: http://github.com/openstack/tempest/commit/820072967e922ee23c8775f0707d1e8a588ade80
Submitter: Jenkins
Branch: master

commit 820072967e922ee23c8775f0707d1e8a588ade80
Author: DennyZhang <email address hidden>
Date: Sun Sep 15 11:57:17 2013 -0500

    add required python modules(six,iso8601,fixtures) to requirements.txt

    These modules of six, iso8601, fixtures, are requiementd by following tempest modules.
    But they are missing in requirements.txt.

     ./tempest/openstack/common/log.py:42:from six import moves
     ./tempest/openstack/common/timeutils.py:25:import iso8601
     ./tempest/api/compute/admin/test_aggregates.py:18:import fixtures
     ./tempest/test.py:22:import fixtures

     Closes-Bug: #1225723

    Change-Id: I5a43559630714c504dba4ef79ba9ee29128dfc69