Fix timezone problems with formprintout

When pusing to for_testrunner_1 branch, we often have this failure on testFormPrintoutAsODT:

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FAIL: test_field_replacement_with_variable (testFormPrintoutAsODT.TestFormPrintoutAsODT)
test variables replacement in ODT documents.
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Traceback (most recent call last):
  File "/srv/slapgrid/slappart9/srv/testnode/zl/soft/4a13de8a0fdf926ddebab0387de0dc5e/parts/erp5/Products/ERP5OOo/tests/testFormPrintoutAsODT.py", line 1252, in test_field_replacement_with_variable
    self.assertEqual(node.text, value.strftime('%d/%m/%Y %H:%M:%S'))
AssertionError: '06/12/2010 11:33:15' != '06/12/2010 11:24:15'

After some investigation, turned out that:

• tests where always failing when running by COMP-2544-3Nodes-ERP5PROJECT2 testnode
• This machine uses SystemV/CST6CDT as timezone
• the test fails in the same way when running locally with TZ=SystemV/CST6CDT

I belive this fixes, I tried running under SystemV/CST6CDT, UTC and Europe/Paris TZ and it passes for all cases.