Add support for creating service / VPN provider files

[jgke]

This requires support from connmand's end, either receiving the configuration folder (/var/lib) or a 'create configuration' dbus method. The former would be more relaxed but requires write permissions, while the latter would be less permissive but also requires only dbus permissions.

[jgke]

VPN api has the VPNManager.Create-dbus function, it should be used.