Key retrieval should use HTTPS instead of HTTP

[config.yaml]
options:
  apt-repository:
    type: string
    default: "deb https://artifacts.elastic.co/packages/5.x/apt stable main"
    description: |
      A deb-line for the apt archive which contains the elasticsearch package.
      This is necessary until elasticsearch gets into the debian/ubuntu archives.
  apt-key-url:
    type: string
    default: "http://packages.elasticsearch.org/GPG-KEY-elasticsearch"
    description: |
      The url for the key for the apt-repository.

apt-key-url can be HTTPS as well as apt-repository.