handle routers without an external gateway in list_router_interfaces
handle routers without an external gateway in list_router_interfaces

list_router_interfaces would fail if called on a router with no external
gateway:

    File "/home/lars/.ansible/tmp/ansible-tmp-1445525437.55-149961620412652/os_router", line 343, in main
      ports = cloud.list_router_interfaces(router, 'internal')
    File "/usr/lib/python2.7/site-packages/shade/__init__.py", line 1922, in list_router_interfaces
      ext_fixed = router['external_gateway_info']['external_fixed_ips']
    TypeError: 'NoneType' object has no attribute '__getitem__'

This commit defaults `ext_fixed` to an empty list if there is no external gateway.

Change-Id: Idbcd5f005ebef47e11377e83a60d2adea7c3380b

openstack/python-openstacksdk,openstack/python-openstacksdk,openstack-infra/shade,stackforge/python-openstacksdk,openstack-infra/shade,stackforge/python-openstacksdk,dtroyer/python-openstacksdk,dtroyer/python-openstacksdk