Generate a NAS by loading an XML file

[dgutierrezATC]

To add the option to load an XML file already generated by OpenNAS that automatically set all the OpenNAS parameters, offering then the possibility to either generate the same NAS or modify the parameters to generate a new one but based in a previous model. This feature will allow people to use the same NAS configuration but avoiding the needed to set manually all the NAS' parameters in OpenNAS.