This is supposed to wait for password data...

(I notice that when it is not available, the property is an empty string, not `nil`)

(coming from #121)

... but instead
```shell
D, [2014-10-01T22:02:35.804411 #3624] DEBUG -- : Waiting for password... 0
E, [2014-10-01T22:02:36.050402 #3624] ERROR -- : Failed waiting for i-bfb993fd: waiter failed
E, [2014-10-01T22:02:36.051406 #3624] ERROR -- : waiter failed (Aws::Waiters::Errors::WaiterFailed)
C:/src/oss/aws-sdk-core-ruby/aws-sdk-core/lib/aws-sdk-core/waiters/waiter.rb:101:in `block in wait'
C:/src/oss/aws-sdk-core-ruby/aws-sdk-core/lib/aws-sdk-core/waiters/waiter.rb:86:in `catch'
C:/src/oss/aws-sdk-core-ruby/aws-sdk-core/lib/aws-sdk-core/waiters/waiter.rb:86:in `wait'
C:/src/oss/aws-sdk-core-ruby/aws-sdk-core/lib/aws-sdk-core/client.rb:75:in `wait_until'
```

based on
```rb
...
def wait_for_password(id)
  logger.debug "Waiting for password for #{id} to be available."
  pwd = @ec2.wait_until(:password_data_available, instance_id: id) do |w|
    w.before_attempt do |a|
      logger.debug "Waiting for password... #{a}"
    end
  end
  logger.info "Password data: #{pwd.password_data}"
  pwd
rescue Aws::Waiters::Errors::WaiterFailed => e
  logger.error "Failed waiting for #{id}: #{e.message}"
  logger.error e
  raise e
end
...
```