Add network monitor for compute node

Nova blueprint 'utilization-aware-scheduling'[1] gave nova scheduler the
ability to schedule VMs based upon transient resource usage like CPU
utilization.

This blueprint proposes to add network monitor to collect network related
metrics on compute node. Resource tracker can get the metrics from network
monitor[2], then nova scheduler can schedule VMs based upon the network
utilization on compute node.

There was effort[3] trying to do this but it was obsoleted, this bp will
continue the work based on it.

[1]https://blueprints.launchpad.net/nova/+spec/utilization-aware-scheduling
[2]https://github.com/openstack/nova/blob/13.0.0.0rc1/nova/compute/resource_tracker.py#L464
[3]https://review.openstack.org/#/c/44007/