Use keyfile to store server configuration

Replace hardcoded server configuration by keyfile
(apertis-hawkbit-agent.ini) with following entries:
  [server]
  base-uri=http://localhost:8080


  #tenant=DEFAULT
  controller-id=<ID>
  key-token=<token>
base-uri, controller-id and key-token are mandatory.

Signed-off-by: Frédéric Danis <frederic.danis@collabora.com>