NetworkManager implementation for os-net-config

Using ifcfg files with the network service for Fedora/CentOS/RHEL has been deprecated. This functionality is no longer enabled by default, will not be available in future versions of the OS. It is being replaced by NetworkManager, so os-net-config should support NetworkManager as a configuration back-end.

Since NetworkManager doesn't have a very capable API, the nmstate python library allows a JSON/YAML declarative configuration for NetworkManager.

The os-net-config library has modular implementations for different OS configurations. There should be a new implementation module in os-net-config that uses nmstate instead of writing ifcfg files. There will be some overlap between the existing ifcfg implementation, so there is an opportunity for some code re-use.