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