Add example of creating per-node os-net-config mappings

Add example of creating per-node os-net-config mappings

Adds an example of proving a mapping file for all nodes, then
extracting the data for each node based on a lookup of the mac address.

Some assumptions are made (e.g the hard-coded reference to eth0), but
it should be easily modified to suit specific environments.

Usage via an enviroment file will look like:

resource_registry:
  OS::TripleO::NodeUserData: os-net-config-mappings.yaml
parameter_defaults:
  NetConfigDataLookup:
    host1:
      nic1: "00:c8:7c:e6:f0:2e"
    host2:
      nic1: "00:18:7d:99:0c:b6"

Note this version requires liberty heat in the undercloud due to the
use of a new str_replace feature to serialize the json parameter.

Change-Id: I7da9c9d8805e676a383e888a7d77f05d3432ab12