When router_distributed=True is set in neutron.conf,
user can't override this and create a centralized router
through openstack client. Openstack client allows modifying
routers from distributed to centralized after creation but
not supporting centralized flag during creation. We allow
centralized and distributed flags during router creation
with this change.
Closes-bug: #1664255
Change-Id: I75f72ca695338ad8c381cfa89fbb9d8e61ee7bc5
fe59e339ae