[BUGFIX beta] Allow tagName to be a computed property.

It will only be used to determine the initial tagName to use, and will not cause a rerender if its value changes.

Closes #9742.
I think we'd have to dig into some gnarly internal chain watcher stuff to know.  Also, `Ember.get` doesn't really indicate if its a computed property or not.

We could however, do something like:

```javascript
var tagName = view.tagName;
if (tagName === undefined) {
  tagName = get(view, 'tagName');
  Ember.deprecate('`tagName` must be specified as a string value.  You specified `' + tagName + '` as a computed property to ' + view + '.', tagName);
}
```

Folks can then get around the deprecation by just setting `tagNam` before the first rendering (either via `on('init', func)` or `init: function() { this.tagName = someComputedValue; }`).