tests: test/vpp_l2.py fix missing name.

test/vpp_l2.py:213:26: F821 undefined name 'L2_VTR_OP'
        self.itf.set_vtr(L2_VTR_OP.L2_DISABLED)

Move L2_VTR_OP enum to vpp_sub_interface.py where the VTR code is found.

Change-Id: I9eb9a3a2c679813c221ce1d0c4fa8aac6076c443
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>
diff --git a/test/test_gre.py b/test/test_gre.py
index 92fc3bc..cee7eea 100644
--- a/test/test_gre.py
+++ b/test/test_gre.py
@@ -10,11 +10,10 @@
 from scapy.volatile import RandMAC, RandIP
 
 from framework import VppTestCase, VppTestRunner
-from vpp_sub_interface import VppDot1QSubint
+from vpp_sub_interface import L2_VTR_OP, VppDot1QSubint
 from vpp_gre_interface import VppGreInterface, VppGre6Interface
 from vpp_ip import DpoProto
 from vpp_ip_route import VppIpRoute, VppRoutePath, VppIpTable
-from vpp_papi_provider import L2_VTR_OP
 from util import ppp, ppc