Support handle sudo password

I've implemented the issue #26

RSpec configuration example

```ruby
# spec/spec_helper.rb
RSpec.configure do |c|
  c.before do
    # some configuration
    c.ssh   = Net::SSH.start(c.host, user, options)
    c.sudo_password = ENV['SUDO_PASSWORD']
  end
end
```

And run test.

```sh
$ SUDO_PASSWORD=foobar rake spec
```