Allow labelling of BMHs

SIP cluster scheduling relies ultimately on labels on the underlying
BareMetalHosts for rack and server name.

This change allows operators to optionally add labels to BMHs
that are generated by the hostgenerator-m3 function, and gives
an example of how to use it.

Change-Id: Icc35ef81fc3a9fc6e6e89dc7ba58a385469cdf68

manifests/function/hostgenerator-m3/hosttemplate.yaml

@@ -47,7 +47,10 @@ template: |
   kind: BareMetalHost
   metadata:
     annotations:
+  {{- if $host.labels }}
     labels:
+  {{ toYaml $host.labels | indent 4 }}
+  {{- end }}
     name: {{ $hostName }}
   spec:
     online: false

manifests/site/test-site/target/catalogues/hosts.yaml

@@ -23,6 +23,8 @@ hosts:
         oam: 52:54:00:9b:27:4c
         pxe: 52:54:00:b6:ed:31
       hardwareProfile: default # defined in the hostgenerator-m3 function
+      labels: # optional
+        airshipit.org/example-label: "label-bmh-like-this"
     node02:
       bootMode: legacy
       macAddress: 52:54:00:b6:ed:02