The --flavor-id option is added to the router create command. The
flavor_id attribute has been supported by the Neutron API for router
POST operations since a long time ago [0].
Partial-Bug: #2020823
[0] https://review.opendev.org/c/openstack/neutron-lib/+/494289
Change-Id: If88499533a92c09e67b0827d1d64156682a3bb8e