Remove default value for username

[edubxb]

Having a default value for the username don't allows to use the ssh_config file (~/.ssh/config) to specify the username used to connect to the instances.

[pmazurek]

Fair point, I'll fix it.

[axitkhurana]

@pmazurek Thanks for the project! I agree with @edubxb would be awesome if the library would let it use ssh_config values by default. Let me know if you'd like a PR on this.

[rszalski]

I think I can speak for @pmazurek that we welcome all PRs, so feel free to submit one!

[pmazurek]

Fixed