OpenVPN: make optional duplicate-cn

[hORn]

Unless the option “duplicate-cn“ on the server side config of OpenVPN is for some reason a must, it would be good to make it optional or remove from the automatic configuration, as there is no way neither to switch it off in GUI, nor to override in custom config.

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[pedro]

OpenVPN: do not add 'duplicate-cn' to server config automatically (resolves #177)

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