Fix NoneType in create_router.py

Fix NoneType in create_router.py

It lets Neutron handle str or unicode instead of returning None.
It reverts the first proposal [1] which raises side effects [2].

[1] https://gerrit.opnfv.org/gerrit/#/c/56171/
[2] https://build.opnfv.org/ci/view/functest/job/functest-compass-baremetal-daily-master/126/console

Conflict:
    snaps/openstack/create_router.py

Change-Id: I1b45c7d6118321dd67343f31253f154abae73655
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>
(cherry picked from commit 9651c889f1b966dc2733daf7eabbb5e62268654d)