Add option to specify client management port for OpenVPN client export use

See forum http://forum.pfsense.org/index.php/topic,63668.0.html and OpenVPN Manager GitHub discussion https://github.com/jochenwierum/openvpn-manager/issues/17
This allows a different client management port to be specified for use by OpenVPN client export when generating a client config for use with OpenVPN manager. Typically a company could have multiple offices with OpenVPN "road-warrior" access. Some users might need to connect to different offices at different times, so they would have multiple OpenVPN client configs installed on their laptop. For this to work with OpenVPN Manager, each client config needs to have a different management channel - only 1 can use the default of "166". The company can choose a different number in the road-warrior server "client parameters" section at each office. Then the generated client config from each office will have a unique management channel port number.

This code was committed to master by https://github.com/pfsense/pfsense/commit/faf61f12deb2ef862be5c3487ea3319bb3aeff0c
The OpenVPN client export utility code was committed by https://github.com/pfsense/pfsense-packages/commit/422243858bab08d21faf97b7d0f5badbc76f8612 - so the client export code is ready and waiting to do this feature once users can specify the data in the base system.
It would be very handy to have it in 2.1.1 so it can be used without manually patching systems.