Add option to have presets provided

[kenkendk]

For setups where multiple machines need to be set up, or to simplify moving configurations between machines, there could be a preset file that is loaded during startup.

This is somewhat similar to --parameters-file but a more system-like setup that takes care of more possible settings that what the --parameters-file is capable of.

The basic outline of such a file could be a JSON structure like the following:

{
  "env": {
    "*": {
      "TEMP": "/mnt/tmp",
      "LOGGING": "false"
   },
   "tray": {
      "LOG": "1"
    },
    "server": {
        "DUPLICATI__WEBSERVICE_ALLOWED_HOSTNAMES": "m1"
    }
  },

  "db": {
    "server": {
      "--compression-module": "zip",
      "--send-http-result-output-format": "Json"
     }
  },

  "args": {
    "tray": [ "--hosturl": "http://m1:8299" ]
    "server" [ "--webservice-port=8299" ]
  }
}

The file would be default placed in the datafolder that Duplicati uses, but should be configureable by an environment variable so the system admin can point to the path location.