network: Adds short sleep between veth setup and neighbour proxy add

network: Adds short sleep between veth setup and neighbour proxy add

There is an intermittent issue, experienced on at least Ubuntu 18.04 (5.3.0-40-generic) and Alpine 3.11 (5.4.12-1-virt) when using the router network interface type that causes the IP proxy neighbour entries on the host side of the veth pair to not be created.

The `ip neigh add proxy` command returns without an error, however by the time the network up hook has started the IP neighbour proxy entries are no longer there (if they ever were).

I've also tested this using netlink rather than the ip command to add and both are equally affected.

Adding a short sleep between setting up the veth pair and adding the proxy entries appears to fix it.

Signed-off-by: Thomas Parrott <thomas.parrott@canonical.com>