Fix BCM SDK tunnel termination object cleanup on vxlan tunnel delete

Ticket: CM-4723
Reviewed By: CCR-2408
Testing Done: vxlan_fld_test.py:test_bond_vxlinux_convergence_flaps, manual testing

On tunnel delete, tunnel termination object in BCM sdk is not cleaned properly.
As a result, mpls_entry in the hardware is left around even though software has
deleted the tunnel. This prevents subsequent dynamic creation of the same tunnel
and also mac learning, as mpls_entry points to invalid SVP with no CML setting
to do hardware learning.

Also, changes in ifupdown2 to make "vxlan-learning on" as default, so that
user/script doesn't have to explicitly turn on the learning.

ifupdownaddons/iproute2.py

@@ -460,7 +460,7 @@ class iproute2(utilsBase):
                           localtunnelip=None,
                           svcnodeips=None,
                           peernodeips=None,
-                          learning='off'):
+                          learning='on'):
         if svcnodeips and peernodeips:
             raise Exception("svcnodeip and peernodeip is mutually exclusive")
         args = ''
@@ -472,9 +472,7 @@ class iproute2(utilsBase):
         if peernodeips:
             for s in peernodeips:
                 args += ' peernode %s' %s
-        if learning == 'on':
-            args += ' learning'
-        else:
+        if learning == 'off':
             args += ' nolearning'
         
         if self.link_exists(name):