multiple NICs could cause cloud-init nonet timeout

when a system pxeboots, we are appending BOOTIF=XX:XX:XX (the mac address that booted).
(Note, that this is done thorugh pxelinux IPAPPEND or changes made under bug 1051867).

cloud-init contains an /etc/network/interfaces file that has an entry for 'eth0'. This entry will cause cloud-init-nonet and static-networking to block boot until a device named 'eth0' appears and is brought up.

The assumption in the cloud image build process is that eth0 will always be dhcp-able, and will contain the link to at least a datasource.

There is an issue with this assumption and the following 2 things:
 a.) pxe booted system with multiple NICs where 'eth0' is not the BOOTIF interface
 b.) kernel biosdevname names interfaces 'en0' or otherwise.

I think i'd like to handle this by adding code that forces a nic to be named eth0 if it is the interface on the command line listed as BOOTIF. If any code makes use of the name before it is renamed, we'd have to rename that codes output as well. Here, I'm thinking of the fact that ipconfig from kblic writes /run/network-$DEVICE.conf . if it had seen the interace as en0 and then we renamed to eth0 we'd have to handle that.