Commits for Bug #321 and Bug #333

vyatta-cfg: return correct path for pppoe or pppoa interfaces

Fix for the _ppp_int function so it returns the actual ethernet
interface associated with the ppp device rather than an empty string.
As a result the path function now returns the correct configuration
path for the interface.

Linked to Bug #321 http://bugzilla.vyos.net/show_bug.cgi?id=321

Bug #333 http://bugzilla.vyos.net/show_bug.cgi?id=333