Fix timezone problems with formprintout

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

```
======================================================================
FAIL: test_field_replacement_with_variable (testFormPrintoutAsODT.TestFormPrintoutAsODT)
test variables replacement in ODT documents.
----------------------------------------------------------------------
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. 

/reviewed-on nexedi/erp5!174

product/ERP5OOo/tests/testFormPrintoutAsODT.py

@@ -1249,7 +1249,7 @@ return []
         self.assertEqual(node.get('{%s}value' % nsmap['office']), str(value))
       elif klass == 'DateTimeField':
         self.assertEqual(node.get('{%s}value-type' % nsmap['office']), 'date')
-        self.assertEqual(node.text, value.strftime('%d/%m/%Y %H:%M:%S'))
+        self.assertEqual(node.text, '06/12/2010 23:24:15')
       elif klass == 'CheckBoxField':
         self.assertEqual(node.get('{%s}value-type' % nsmap['office']),
                           'boolean')

product/Formulator/Widget.py

@@ -1629,8 +1629,7 @@ class DateTimeWidget(Widget):
       # default offset is 30/12/1899
       number_of_second_in_day = 86400 #24 * 60 * 60
       timestamp = float(value)
-      # XXX Works only if the timezone is the same in OpenOffice
-      ooo_offset_timestamp = float(DateTime(1899, 12, 30))
+      ooo_offset_timestamp = float(DateTime(1899, 12, 30, 0, 0, 0, value.timezone()))
       days_value = (timestamp - ooo_offset_timestamp) / number_of_second_in_day
       attr_dict['{%s}formula' % TEXT_URI] = 'ooow:%f' % days_value
       text_node.attrib.update(attr_dict)