pppoe: make pppoe plugin work with dot1q subinterfaces

pppoe: make pppoe plugin work with dot1q subinterfaces

- Enabling arc "device-input" with the next node "pppoe-input" on the pppoe cp interface: to get rid of L3_MAC_MISMATCH error
- Parsing in the "pppoe-input" node fixed to parse all headers from the scratch
- Getting mac address directly from encap interface when filling up DPO adjacency. Anyway, in the case of the dot1q subinterface, we need to get vlan tags to fill the DPO adjacency.

Type: improvement

Change-Id: I5405931b0f58bef7b852c079a7e66e0da8b5de0f
Signed-off-by: Stanislav Zaikin <zstaseg@gmail.com>