Configuration parser should handle quotes

If you provide a path to a file or directory in graylog.conf that contains quotes, like
`message_journal_dir = "data/journal"`
we will use them literally for the directory.
This can lead to some head scratching.
Sometimes graylog will create directories with quotes: `./"data/journal"/messagejournal-0`,
or fail with an error message that isn't obvious:
> com.github.joschi.jadconfig.ValidationException: Cannot execute integrations_scripts_dir at path "/var/tmp". Please specify the correct path or change the permissions

## Expected Behavior
We should either interpret quoted strings, or reject them while parsing the configuration.

* Graylog Version: 3.0-rc1