lxd/network/driver/ovn: Only add default route and SNAT rules to router after adding external router port
lxd/network/driver/ovn: Only add default route and SNAT rules to router after adding external router port

Fixes log message "No path for static route 0.0.0.0/0; next hop n.n.n.n" when creating/editing network.

Also only add default routes and SNAT rules if logical router has external IPs.

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

hallyn/lxd,lxc/lxd,stgraber/lxd,lxc/lxd,lxc/lxd,hallyn/lxd,stgraber/lxd,atxwebs/lxd,lxc/lxd,stgraber/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,lxc/lxd,hallyn/lxd,lxc/lxd,atxwebs/lxd,atxwebs/lxd,atxwebs/lxd,hallyn/lxd,stgraber/lxd,hallyn/lxd,hallyn/lxd,stgraber/lxd