lxd/network/driver/ovn: Update ovnNetworkExternalSubnets to return externalSubnetUsage
lxd/network/driver/ovn: Update ovnNetworkExternalSubnets to return externalSubnetUsage

And also removes the check that each used subnet is valid in the uplink's routes as this shouldve been checked before getting into the DB.

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

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