实例化alertcontroller之后，可以遍历view的子视图，子视图title和message嵌套了多层父视图，（需要遍历6层subViews），
下面是取title和message的父视图的代码：
UIView *subView1 = alert.view.subviews[0];
UIView *subView2 = subView1.subviews[0];
UIView*subView3 = subView2.subviews[0];
UIView*subView4 = subView3.subviews[0];
UIView*subView5 = subView4.subviews[0];
取title和message：
UILabel*title = subView5.subviews[0];
UILabel*message = subView5.subviews[1];
然后设置message内容居左：
message.textAlignment = NSTextAlignmentLeft;
swift版：
let alert =UIAlertController(title:"版本升级", message: message,preferredStyle: .Alert)
let subView = alert.view.subviews[0].subviews[0].subviews[0].subviews[0].subviews[0]
let msgLabel = subView.subviews[1]as?UILabel
msgLabel?.textAlignment= .Left
