Remove SNAPS test_create_router_external_network

Remove SNAPS test_create_router_external_network

test_create_router_external_network fails if DVR.
Then CreateRouterSuccessTests has to be removed due to the interface
between Functest and SNAPS.

ft1.5: snaps.openstack.tests.create_router_tests.CreateRouterSuccessTests.test_create_router_external_network_StringException: Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/snaps/openstack/tests/create_router_tests.py", line 431, in test_create_router_external_network
    self.check_router_recreation(router, router_settings)
  File "/usr/lib/python2.7/site-packages/snaps/openstack/tests/create_router_tests.py", line 499, in check_router_recreation
    len(derived_settings.port_settings))
  File "/usr/lib/python2.7/unittest/case.py", line 513, in assertEqual
    assertion_func(first, second, msg=msg)
  File "/usr/lib/python2.7/unittest/case.py", line 506, in _baseAssertEqual
    raise self.failureException(msg)
AssertionError: 1 != 4

Change-Id: I09946de01b5d0f0c9b9a41a6520d6b2be51f5238
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>