Fix assembly of /etc/network/interfaces

Merged Jake Buchholz Göktürk requested to merge fix/interface_assembly into master

Include the other interface configs from /etc/network/interfaces.d/ after lo and existing eth*, as was initially intended.

Also separate out the assembly code into its own script, as this is done both on boot via eth-eni-setup and on hotplug events by eth-eni-hotplug.

Resolves #91 (closed)

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