node-proxy: create entrypoint main()

node-proxy: create entrypoint main()

This creates a `main()` function in server.py that will be the
entrypoint of node-proxy.

This also implement arg parsing and add a `--config` parameter
to specify the configuration file.

Finally, this introduce a small refactor of class `Config` and class
`Logger` in util.py because there was a circular dependency between them.

Signed-off-by: Guillaume Abrioux <gabrioux@ibm.com>