Skipped vxlan test cases in sonic-mgmt

[yue-fred-gao]

The following tests are skipped due to error/failure. Needs investigation to find out if sonic-vpp can support them.

vxlan/test_vxlan_ecmp.py::Test_VxLAN_ecmp_random_hash
vxlan/test_vxlan_ecmp.py::Test_VxLAN_entropy
vxlan/test_vxlan_ecmp.py::Test_VxLAN_underlay_ecmp
vxlan/test_vxlan_bfd_tsa.py