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_vtr.py b/test/test_vtr.py
index d69dd20..63e1cc7 100644
--- a/test/test_vtr.py
+++ b/test/test_vtr.py
@@ -9,8 +9,7 @@
 
 from util import Host
 from framework import VppTestCase, VppTestRunner
-from vpp_sub_interface import VppDot1QSubint, VppDot1ADSubint
-from vpp_papi_provider import L2_VTR_OP
+from vpp_sub_interface import L2_VTR_OP, VppDot1QSubint, VppDot1ADSubint
 from collections import namedtuple
 
 Tag = namedtuple('Tag', ['dot1', 'vlan'])