Issue title: Diffie Hellman Parameters
 Issue body: I like that this supports generating 4096 bit RSA keys. It would be good if the scripts could be enhanced to also generate new 4096 DH parameters. I'm not sure how to incorporate this into the scripts, but the OpenSSL command would be: `openssl dhparam 4096 -out pki/dh.pem`.

For now, my work around is to generate these manually after running `ovpn_copy_server_files` on my private CA box. 

By the way, thanks for the great image and good documentation!
 Comments: 
 Comment 0: Disregard. Just realized this is already handled. 
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