NMVpnPluginInfo is little more then a wrapper around
the GKeyFile that describes the VPN plugin settings,
i.e. the name files under "/etc/NetworkManager/VPN/".

Add this class to make the VPN API more explicit. Clients
now can use NMVpnPluginInfo instead of concerning themselves
with loading the keyfile and the meaning of its properties.

Also add support for a new VPN plugins directory
"/usr/lib/NetworkManager/VPN", which should replace
"/etc/NetworkManager/VPN" in the future. But we have to
consider both locations for backward compatibility.

The content of the VPN directory is not user configuration,
hence it should not be under "/etc". See related bug 738853.