Add default port

[dcastro]

Clarification and motivation

As of #126, the servant server now requires the user to specify a port (either via a command line option or an env var).

It could be useful to use some default port (say, 8081) when neither a command line option nor an env var are set, to make it easier to start the server during development.

Acceptance criteria

• The server runs on port 8081 when a command line option or an env var are not set.