Additional ENIs attached/detached pre-boot need to be configured.

(just when you thought we were done with this!)

Discovered an edge case late last night. While #85 takes care of IPv6 and secondary IPv4's via udhcpc hooks, and hot un/plugging ENIs is taken care of via mdev config/script, an instance that has an extra ENI attached before the instance is started is not set up.

We should be able to accomplish this by ensuring /sys/class/net/eth* has configuration in /etc/network/interfaces before the networking service comes up, similar to what we do for mdev hotplugging. Then, when networking service comes up, all ENIs should be configured.