oo-accept-node: Add check_ext_net_dev_addr

Add `check_ext_net_dev_addr` to `oo-accept-node` to check that the network interface specified by `EXTERNAL_ETH_DEV` in `/etc/openshift/node.conf` has a globally scoped IPv4 address (if it does not, `oo-iptables-port-proxy` will fail).

This commit fixes bug 1121206.