Remove NetworkManager conflict for EL7+
Merge pull request #40 from szarkos/2.0

Remove NetworkManager conflict for EL7+
Various fixes for CoreOS distro

- Clean up service unit
- Add /etc/machine-id to deprovision blacklist
- Use `ip link` instead of `ifup`/`ifdown`
- Stop networkd before running DHCP client
Refactor DHCP start/stop into its own methods