Add PPPoE connections

Created by: steglicd

This Pull request will add the functionality to configure PPPoE based connections via Plinth UI. This type of connections are used by ISP's to connect the customers to the internet.

To get the feature working two packages must be installed first: apt-get install ppp pppoe

I have tested this and was able to set up a working PPPoE connection. I have two little Problems:

Activate / Deactivate Button behaves strange

- when PPPoE Discovery times out, the connection is shown as "Not Active" on the network overview page. After some time networkmanager retry to establish the connection. During this phase, the connection is shown as "Active" again. - When the connection is up and running and I click on "Deactivate" a new connection appears automatically (the name of the new connection is the name of the linux device - like "eth1"). When re-activating the PPPoE connection, the "eth1" connection disappears again. So no functional restriction.

Password storage

The NetworkManager does not give me back my PPPoE Password. When I bringup a PPPoE connection and run on python console:

client = nm.Client.new(None)
c = client.get_active_connections()
ppp = client.get_connection_by_uuid(c[0].get_uuid())
settings = ppp.get_setting_pppoe()
settings.get_password()

I get a empty string. I don't know if this is security feature, a bug or if I'm simply to stupid to get it. The result of this issue is, that if you edit a PPPoE connection, you get a blank password field which is marked to be incomplete.