helm: Add a configmap entry for aptly.conf

Usage (in values.yaml):

global:
  backend:
    aptly:
      rootDir: /srv/obs/aptly
      ppaDistributorID: ""

This generates:

{
  "ppaDistributorID": "",
  "rootDir": "/srv/obs/aptly"
}

at the location /etc/obs/aptly.conf.

cc @sjoerd, @adalessandro

helm/open-build-service/charts/backend/templates/configmap.yaml

@@ -8,3 +8,4 @@ data:
         default "" .Values.global.backend.extraConfig
       ) | quote
     }}
+  aptly.conf: {{ default "" .Values.global.backend.aptly | toPrettyJson | quote }}