Allow to override upstream user (#7)

The syntax is <upstream_id>[_<user>], e.g.,

* log in as the default job user:

    3b07781fc52d4427b3f4e83f16abb104@ssh.dstack.example.com

* log in as `root`:

    3b07781fc52d4427b3f4e83f16abb104_root@ssh.dstack.example.com