OUs are not using CN but ou field and has no restriction on valid charac...

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From what I saw at http://support.microsoft.com/kb/909264 OUs names have no restriction on the characters used for its name (ou field) so is a bug to apply the same restrictions a CN has. This should fix the problem when activating zentyal-mail. I didn't run anste for this change because my laptop is busy testing another branch and I think this change is safe to merge.