OVH network doesn't seem to announce an IPv6 route for a VM at first.
If from the VM, we reach another network, the route is then
announced for a while, before being dropped.
To workaround that behavior, a solution could be to ping regularly
an external site so packets reach OVH router and a route is
announced.
This fix is needed on web-001 for Alkane. Ref T1828.