VPP-35 Add GRE tunnels to sw_interface_dump

VPP-35 Add GRE tunnels to sw_interface_dump

GRE tunnels were missing from vpp_api_test's sw_interface_dump command.

$ sudo ./build-root/install-vpp_debug-native/vpp-api-test/bin/vpp_api_test json
vat# sw_interface_dump
[
...
  {
    "sw_if_index": 7,
    "sup_sw_if_index": 7,
    "l2_address_length": 0,
    "l2_address": [0, 0, 0, 0, 0, 0, 0, 0],
    "interface_name": "gre0",
    "admin_up_down": 1,
    "link_up_down": 0,
    "link_duplex": 0,
    "link_speed": 0,
    "mtu": 0,
    "sub_id": 0,
    "sub_dot1ad": 0,
    "sub_number_of_tags": 0,
    "sub_outer_vlan_id": 0,
    "sub_inner_vlan_id": 0,
    "sub_exact_match": 0,
    "sub_default": 0,
    "sub_outer_vlan_id_any": 0,
    "sub_inner_vlan_id_any": 0,
    "vtr_op": 0,
    "vtr_push_dot1q": 0,
    "vtr_tag1": 0,
    "vtr_tag2": 0
  }
]

Change-Id: I6f174f3c384eac464250b22f43b25be6a844aa66
Signed-off-by: Chris Luke <chrisy@flirble.org>