Ship systemd unit files and update them through racker, bootstrap: drop pxe_interface file and use the node.csv instead

[pothos]

• bootstrap: drop pxe_interface file and use the node.csv instead The PXE interface can be found automatically by looking into the column of secondary MAC addresses and searching for the one of the current node which is the management node.
• Ship systemd unit files and update them through racker The systemd unit files were generated by the bootstrap process which meant that the container images were not covered by "racker update". Ship the unit files and create symlinks for them, restarting the services as needed on extraction.

[joaquimrocha]

It LGTM so far. If you made it WIP, then maybe you want to keep adding things here. Otherwise, feel free to merge.

[pothos]

Tested it with various scenarios of having existing service files or not