bugfix: do not allocate pty on ssh by default

PTY will effectively disable stderr output, so avoid it for now.

It will come up very soon when we get back to sudo; see if fifo or other solutions might be used. Stderr is important for accurate command execution...

For reference see this wonderful explanation:

http://unix.stackexchange.com/a/134169
Signed-off-by: Dominik Richter <dominik.richter@gmail.com>

lib/vulcano/backend/ssh.rb

@@ -24,9 +24,6 @@ module Vulcano::Backends
       cmd.force_encoding('binary') if cmd.respond_to?(:force_encoding)
 
       @ssh.open_channel do |channel|
-        channel.request_pty do |_, success|
-          abort 'Could not obtain pty on SSH channel ' if !success
-        end
         channel.exec(cmd) do |_, success|
           unless success
             abort 'Couldn\'t execute command on SSH.'