Allow customization of keys_only & paranoid SSH

This adds two new SSH configuration options:

- `keys_only`
- `paranoid`

These values were previously hard-coded, but can now be user-specified.

Fixes GH-4275

lib/vagrant/machine.rb

@@ -434,6 +434,8 @@ module Vagrant
       info[:host] ||= @config.ssh.default.host
       info[:port] ||= @config.ssh.default.port
       info[:private_key_path] ||= @config.ssh.default.private_key_path
+      info[:keys_only] ||= @config.ssh.default.keys_only
+      info[:paranoid] ||= @config.ssh.default.paranoid
       info[:username] ||= @config.ssh.default.username
 
       # We set overrides if they are set. These take precedence over

plugins/communicators/ssh/communicator.rb

@@ -335,8 +335,8 @@ module VagrantPlugins
           forward_agent:         ssh_info[:forward_agent],
           send_env:              ssh_info[:forward_env],
           keys:                  ssh_info[:private_key_path],
-          keys_only:             true,
-          paranoid:              false,
+          keys_only:             ssh_info[:keys_only],
+          paranoid:              ssh_info[:paranoid],
           password:              ssh_info[:password],
           port:                  ssh_info[:port],
           timeout:               15,

plugins/kernel_v2/config/ssh_connect.rb

@@ -7,6 +7,8 @@ module VagrantPlugins
       attr_accessor :username
       attr_accessor :password
       attr_accessor :insert_key
+      attr_accessor :keys_only
+      attr_accessor :paranoid
 
       def initialize
         @host             = UNSET_VALUE
@@ -15,6 +17,8 @@ module VagrantPlugins
         @username         = UNSET_VALUE
         @password         = UNSET_VALUE
         @insert_key       = UNSET_VALUE
+        @keys_only        = UNSET_VALUE
+        @paranoid         = UNSET_VALUE
       end
 
       def finalize!
@@ -24,6 +28,8 @@ module VagrantPlugins
         @username         = nil if @username == UNSET_VALUE
         @password         = nil if @password == UNSET_VALUE
         @insert_key       = true if @insert_key == UNSET_VALUE
+        @keys_only        = true if @keys_only == UNSET_VALUE
+        @paranoid         = false if @paranoid == UNSET_VALUE
 
         if @private_key_path && !@private_key_path.is_a?(Array)
           @private_key_path = [@private_key_path]

website/source/docs/vagrantfile/ssh_settings.html.md

@@ -62,6 +62,16 @@ the machine, but replace it with perhaps a more secure key later.
 
 <hr>
 
+`config.ssh.keys_only` - Only use Vagrant-provided SSH private keys (do not use
+any keys stored in ssh-agent). The default value is `true`.`
+
+<hr>
+
+`config.ssh.paranoid` - Perform strict host-key verification. The default value
+is `true`.
+
+<hr>
+
 `config.ssh.forward_agent` - If `true`, agent forwarding over SSH
 connections is enabled. Defaults to false.