While fixing https://bugs.launchpad.net/neutron/+bug/1876148 we've made
the add_router_interface method idempotent and it no longer raised a
Conflict if the same interface was added twice, this broke the
test_add_interface_in_use from neutron-tempest-pluign.
Change-Id: I76d46a4c3c8af166e022894a5693d5c78555ea50
Closes-Bug: #1895671
Signed-off-by: Lucas Alvares Gomes <lucasagomes@gmail.com>