Extract logic for current target ssh user name:

  Add Remote#user_name method, with code returning which user name
will be used for SSH authentication.
Implement Remote#config:

  Load SSH configuration for targeted host name.
Use SSH user name configured for a given target:

  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.