[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.

i think its just  `!this.metaForPropertery('tagName')._dependentKeys`