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.

features/steps/etc_steps.rb

@@ -0,0 +1,5 @@
+# FIXME: our monkey patch currently prevent us from using `must' in step
+# definitions.
+Then(/^the output should contain my current login name$/) do
+  assert_partial_output(Etc.getlogin, all_output)
+end

features/steps/ssh_steps.rb

@@ -0,0 +1,6 @@
+# FIXME: current home directory shouldn't be changed here, maybe we should use
+# a tag for features needing a fake home directory.
+Given(/^an SSH config with:$/) do |config|
+  ENV['HOME'] = File.expand_path current_dir
+  write_file '.ssh/config', config
+end