Add an m3 host generator function

This PS has a function which constructs a collection of Metal3 BareMetalHost
resources, along with associated configuration Secrets.
It solves for a couple of things:

1. pulling the nitty gritty details for generating BMH into one reusable place,
2. allowing the site-specific details to be filled in via catalogues of values

This function leverages a couple of different plugins in sequence:
The airshipctl Replacement plugin, which pulls the site-specific data from
the catalogue documents into a Templater plugin configuration; and then
the airshipctl Templater plugin, which generates a variable number of
BMHs in a data-driven fashion.

More details can be found in the README.md in this patchset.

Closes: #245

Change-Id: I3ddbd36dc53ea6afbd633098c985f4b28bcbb793

manifests/type/gating/shared/catalogues/common-networking.yaml

@@ -0,0 +1,39 @@
+# Type-level networking catalogue.  This info feeds the Templater
+# kustomize plugin config in the hostgenerator-m3 function.
+apiVersion: airshipit.org/v1alpha1
+kind: VariableCatalogue
+metadata:
+  name: common-networking-catalogue
+commonNetworking:
+  links:
+    - id: oam
+      name: enp0s3
+      type: phy
+      mtu: "1500"
+      # ethernet_mac_address: <from host-catalogue> (optional)
+    - id: pxe
+      name: enp0s4
+      type: phy
+      mtu: "1500"
+      # ethernet_mac_address: <from host-catalogue> (optional)
+  networks:
+    - id: oam-ipv4
+      type: ipv4
+      link: oam
+      # ip_address: <from host-catalogue>
+      netmask: 255.255.255.0
+      routes:
+        - network: 0.0.0.0
+          netmask: 0.0.0.0
+          gateway: 10.23.25.1
+    - id: pxe-ipv4
+      type: ipv4
+      link: pxe
+      # ip_address: <from host-catalogue>
+      netmask: 255.255.255.0
+  services:
+    - address: 8.8.8.8
+      type: dns
+    - address: 8.8.4.4
+      type: dns
+

manifests/type/gating/shared/catalogues/kustomization.yaml

@@ -0,0 +1,4 @@
+apiVersion: kustomize.config.k8s.io/v1beta1
+kind: Kustomization
+resources:
+  - common-networking.yaml