Allow requesting a PTY

This change allows the user to request a pseudo-tty:

```
require 'train'
t = Train.create('ssh', host: '127.0.0.1', port: '2200', user: 'vagrant', password: 'vagrant', sudo: true, pty: true);
puts t.connection.run_command("ls /")
```

While this allows passing the `requiretty` sudoer defaults of certain
RedHat-ish distributions, it comes at the cost of having stderr and
stdout merged together. The change includes a warning for this.

The command above, for example, outputs:

```
<struct Train::Extras::CommandResult stdout="/etc/profile.d/lang.sh: line 19: warning: setlocale: LC_CTYPE: cannot change locale (UTF-8): No such file or directory\r\nbin dev  home  lib64\tmnt  proc  run\t shared  sys  usr      var\r\nboot  etc  lib\t media\topt  root  sbin  srv\t tmp  vagrant\r\n", stderr="", exit_status=0>
```

For comparison, this is the output _without_ the PTY:

```
<struct Train::Extras::CommandResult stdout="bin\nboot\ndev\netc\nhome\nlib\nlib64\nmedia\nmnt\nopt\nproc\nroot\nrun\nsbin\nshared\nsrv\nsys\ntmp\nusr\nvagrant\nvar\n", stderr="/etc/profile.d/lang.sh: line 19: warning: setlocale: LC_CTYPE: cannot change locale (UTF-8): No such file or directory\n", exit_status=0>
```

Note how `ls` behaves differently -- in the presence of a PTY, it will
prettify its output.

lib/train/transports/ssh.rb

@@ -27,6 +27,7 @@ module Train::Transports
   #
   # @author Fletcher Nichol <[email protected]>
   class SSHFailed < Train::TransportError; end
+  class SSHPTYFailed < Train::TransportError; end
 
   # A Transport which uses the SSH protocol to execute commands and transfer
   # files.
@@ -55,6 +56,7 @@ class SSH < Train.plugin(1)
     option :connection_retry_sleep, default: 1
     option :max_wait_until_ready, default: 600
     option :compression, default: false
+    option :pty, default: false
 
     option :compression_level do |opts|
       # on nil or false: set compression level to 0
@@ -98,6 +100,10 @@ def validate_options(options)
         options[:auth_methods].push('password', 'keyboard-interactive')
       end
 
+      if options[:pty]
+        logger.warn('[SSH] PTY requested: stderr will be merged into stdout')
+      end
+
       super
       self
     end

lib/train/transports/ssh_connection.rb

@@ -79,6 +79,12 @@ def run_command(cmd)
         # wrap commands if that is configured
         cmd = @cmd_wrapper.run(cmd) unless @cmd_wrapper.nil?
 
+        if @transport_options[:pty]
+          channel.request_pty do |_ch, success|
+            raise Train::Transports::SSHPTYFailed, "Requesting PTY failed" unless success
+          end
+        end
+
         channel.exec(cmd) do |_, success|
           abort 'Couldn\'t execute command on SSH.' unless success
 

test/unit/transports/ssh_test.rb

@@ -42,6 +42,10 @@ def detect_family
     it 'has default user' do
       ssh.options[:user].must_equal 'root'
     end
+
+    it 'by default does not request a pty' do
+      ssh.options[:pty].must_equal false
+    end
   end
 
   describe 'opening a connection' do
@@ -83,7 +87,7 @@ def detect_family
   end
 
   describe 'converting connection to string for logging' do
-   it "masks passwords" do
+   it 'masks passwords' do
       assert_output(/.*:password=>"<hidden>".*/) do
         connection = cls.new(conf).connection
         puts "#{connection}"
@@ -96,7 +100,7 @@ def detect_family
       cls.new(conf).connection
     end
 
-    it 'doesnt like host == nil' do
+    it 'does not like host == nil' do
       conf.delete(:host)
       proc { cls.new(conf).connection }.must_raise Train::ClientError
     end
@@ -106,13 +110,13 @@ def detect_family
       cls.new(conf).connection.method(:options).call[:user] == 'root'
     end
 
-    it 'doesnt like key and password == nil' do
+    it 'does not like key and password == nil' do
       conf.delete(:password)
       conf.delete(:key_files)
       proc { cls.new(conf).connection }.must_raise Train::ClientError
     end
 
-    it 'wont connect if its not possible' do
+    it 'wont connect if it is not possible' do
       conf[:host] = 'localhost'
       conf[:port] = 1
       conn = cls.new(conf).connection