add option backup_file_format in config

- #83 Fix tarfile options for python2
- #64 Remove empty folders when backup file (.tar.gz) created.

This commit introduces the settings option client_cert. It is now
possible to add a file path to a client certificate in the configuration
file or as an environment variable. The default is None.

* **config file:**

```
{
  "general": {
    ...
    "client_cert": "/home/user/.certs/grafana.pem"
  },
  ...
}
```

* **env variable:** CLIENT_CERT

Signed-off-by: Jürgen Löhel <juergen.loehel@inlyse.com>

* Remove ability to use yaml for configuration in favor of json
* Remove conf module
* README.me updates

* replaced pathlib with os.path
* added JSON config support

* refactoring to use positional statements

* refactored, realized argparse is being used in individual commands...
* added backup Makefile
* removed backup_grafana.sh and restore_grafana.sh
* removed docker_entry.sh
* (Need to update README and CHANGELOG...)
* (Need to add support for --verbose)