[RFE] NetworkManager implementation for os-net-config

Description of problem:
The network init-scripts (including ifcfg files in /etc/sysconfig/network-scripts) are deprecated in favor of NetworkManager. In order to migrate away from init-scripts in os-net-config, we need to support a NetworkManager implementation in addition to the ifcfg implementation.

Version-Release number of selected component (if applicable):
OpenStack Stein

Actual results:
Currently all interface configuration is done in ifcfg files, and additionally there is a script to configure SR-IOV virtual functions at boot time. os-net-config also has support for ethtool commands and ovs-vsctl commands for OVS.

Expected results:
In order to switch to a NetworkManager implementation, we will use an interface to NetworkManager such as nmcli, nmstate, or Ansible Networking. Additionally, NetworkManager will have to have feature parity for interface types supported by os-net-config.

Additional info:
Most of the interface types are supported in NetworkManager, but we should confirm support in NetworkManager for the following device types:

OVS bonds
OVS patches (internal patches between bridges)
OVS tunnel ports
OVS User bridge (DPDK bridge)
OVS DPDK ports
OVS DPDK bonds
Infiniband network interfaces
VPP interfaces
VPP bonds
Contrail vRouter
Contrail vRouter DPDK
Big Switch IVS bridges
Big Switch NFVSwitch bridges (DPDK)
Big Switch NFVSwitch ports (DPDK)
Big Switch NFVSwitch internal patches (DPDK)

In addition, SR-IOV VF virtual functions are recently supported in NetworkManager, but we should confirm wheter the os-net-config SR-IOV configuration script that gets run by systemd is required when using NetworkManager to manage SR-IOV VFs.