Codegen for netconf merge

[jgroom33]

It should be possible to generate the code for def _state_merged method

Two additional pieces of information are required in order to accomplish the codegen

1. singular entity name
2. key name

Proposal:

Add RESOURCE_KEY_NAMEand RESOURCE_SINGULAR to template:

---

NETWORK_OS: myos
RESOURCE: interfaces
RESOURCE_SINGULAR: interface
RESOURCE_KEY_NAME: name
COPYRIGHT: Copyright 2019 Red Hat
LICENSE: gpl-3.0.txt

This would allow the template to be:

    def _state_merged(self, want, have):
        """ The command generator when state is merged

        :rtype: A list
        :returns: the xml necessary to migrate the current configuration
                  to the desired configuration
        """
        {{ resource }}_xml = []
        for child in want:
            {{ resource }} = build_root_xml_node("{{ resource }}")
            {{ resource_singular }}= build_child_xml_node({{ resource }}, "{{ resource_singular }}")
            build_child_xml_node(
                {{ resource_singular }}, "{{ resource_key_name }}", child["{{ resource_key_name }}"]
            )

        return {{ resource }}_xml

Additional codegen could be implemented based on the schema of the resource.