Complete docs for command line flags

* Split flags into required and optional. Currently only `--master` is
  required.
* Alphabetize the list of optional flags for easier scanning.
* List current defaults for all options as defined in
  `MarathonConf.scala`.
* Add missing options “artifact_store” and “reconciliation_interval”.

Addresses part of issue #432.
Also missing: options inherited from Chaos (see [HttpConf.scala](https://github.com/mesosphere/chaos/blob/master/src/main/scala/mesosphere/chaos/http/HttpConf.scala)):

- `http_port`
- `https_port`
- `ssl_keystore_path`
- `ssl_keystore_password`
- `http_credentials`
- `assets_path`