Add nmcli implementation to os-net-config for Atomic

The os-net-config utility currently supports two different implementations of network configuration. The impl_ifcfg implementation writes out ifcfg- and route- files to /etc/sysconfig/network-scripts, while the impl_eni writes out the /etc/network/interfaces configuration file. An Atomic host is a container host with minimal user environment. There are no /etc/network/interfaces or /etc/sysconfig/network-scripts directories, instead network configuration must be done with nmcli commands. In order to support this model, os-net-config should implement impl_nmcli, which will create a script to run the proper nmcli commands and ensure that the script gets run on startup.