Add support for a no_change option in command module

A new argument `no_change=yes` can be passed to the command module which forces the return status of `change=False`.
This is useful for commands you are sure are not going to modify anything and in combination with register. For example:

    - action: command ls / no_change=yes
      register: root_files
    
    #do something with ${root_files.stdout}