add method get_build_stages, obtain builded stages info for a job

[zousu]

irb(main):007:0* ap @client.job.get_build_stages("a", 4)

I, [2018-06-28T13:57:41.191548 #3445] INFO -- : Obtain builded stages info of 'a' Build #4 { "_links" => { "self" => { "href" => "/job/a/4/wfapi/describe" } }, "id" => "4", "name" => "#4", "status" => "SUCCESS", "startTimeMillis" => 1530157134363, "endTimeMillis" => 1530157138222, "durationMillis" => 3859, "queueDurationMillis" => 51, "pauseDurationMillis" => 0, "stages" => [ [0] { "_links" => { "self" => { "href" => "/job/a/4/execution/node/6/wfapi/describe" } }, "id" => "6", "name" => "Checkout", "execNode" => "", "status" => "SUCCESS", "startTimeMillis" => 1530157135263, "durationMillis" => 826, "pauseDurationMillis" => 0 }, [1] { "_links" => { "self" => { "href" => "/job/a/4/execution/node/12/wfapi/describe" } }, "id" => "12", "name" => "Build", "execNode" => "", "status" => "SUCCESS", "startTimeMillis" => 1530157136212, "durationMillis" => 234, "pauseDurationMillis" => 0 }, [2] { "_links" => { "self" => { "href" => "/job/a/4/execution/node/17/wfapi/describe" } }, "id" => "17", "name" => "Test", "execNode" => "", "status" => "SUCCESS", "startTimeMillis" => 1530157136576, "durationMillis" => 259, "pauseDurationMillis" => 0 }, [3] { "_links" => { "self" => { "href" => "/job/a/4/execution/node/22/wfapi/describe" } }, "id" => "22", "name" => "Deploy", "execNode" => "", "status" => "SUCCESS", "startTimeMillis" => 1530157136999, "durationMillis" => 223, "pauseDurationMillis" => 0 }, [4] { "_links" => { "self" => { "href" => "/job/a/4/execution/node/27/wfapi/describe" } }, "id" => "27", "name" => "Version", "execNode" => "", "status" => "SUCCESS", "startTimeMillis" => 1530157137354, "durationMillis" => 546, "pauseDurationMillis" => 0 } ] } => nil