Support for North American DOCSIS Standard

[NinoRy]

As an ISP could operate on different continents using both standards (NA and EU) we have to add an option to chose the standard in the Configfile. Depending on that the configfile must be built with the appropriate parameter by the docsis tool.

Note: This is only necessary for MTA. So this option should only appear in Edit view when type MTA is chosen.

We also should add a field in Global Configuration -> Provisioning so that it's not necessary to specify it everytime. If not specified the global config value is used to determine the parameter.