With Nebula 1.4.0, if you create an unsafe_route that has a collision with an existing route on the system, the unsafe_route will be silently dropped (and the existing system route remains).
With Nebula 1.5.0, this same situation will cause Nebula to fail to start with an error (EEXIST).
This change restores the Nebula 1.4.0 behavior (but with a WARN log as well).
With the previous implementation, we check if route.MTU is greater than zero,
but it will always be because we set it to the default MTU in
parseUnsafeRoutes. This change leaves it as zero in parseUnsafeRoutes so
it can be examined later.
