core: warn if password only on `vagrant ssh`

lib/vagrant/action/builtin/ssh_exec.rb

@@ -35,6 +35,10 @@ module Vagrant
             end
           end
 
+          if info[:private_key_path].empty? && info[:password]
+            env[:ui].warn(I18n.t("vagrant.ssh_exec_password"))
+          end
+
           # Exec!
           SSH.exec(info, env[:ssh_opts])
         end

templates/locales/en.yml

@@ -77,6 +77,11 @@ en:
       %{names}
     provisioner_cleanup: |-
       Running cleanup tasks for '%{name}' provisioner...
+    ssh_exec_password: |-
+      The machine you're attempting to SSH into is configured to use
+      password-based authentication. Vagrant can't script entering the
+      password for you. If you're prompted for a password, please enter
+      the same password you have configured in the Vagrantfile.
 
     cfengine_config:
       classes_array: |-