When a user is configured for the targeted host name, use it for SSH
authentification instead of the name of current user logged in.
* Modify Remote#user_name to use configured user name.
Currently the idea is to later implement a -h (or -t) option to the
command, and/or a `target' recipe keyword. Another option might be to
use the target/host key when the key/value registry will be implemented
and made available to the recipe and task DSLs.
