Auto merge of #6600 - ember-cli:clean-root-url, r=rwjblue
Auto merge of #6600 - ember-cli:clean-root-url, r=rwjblue

Apply clean-base-url to config.rootURL

So here's the deal: this is an improvement but it is actually a change. This method has not changed:
- [In 2.4, before all `baseURL` changes.](https://github.com/ember-cli/ember-cli/blob/v2.4.3/lib/broccoli/ember-app.js#L1656-L1663)
- [In `master` after changes.](https://github.com/ember-cli/ember-cli/blob/9e674547c82fa6747895e2b5b7f6a53e73d0226f/lib/broccoli/ember-app.js#L1752-L1759)

We would populate the `base` tag with `/` if the provided value was falsey, but the config value would always be the original as specified by the user.

[Flipping through the switchover commit you can see that we didn't adjust what appears in the config module in any way.](https://github.com/ember-cli/ember-cli/pull/5792)

I'm in favor of this change since this is what we do internally for all usages. Exposing it to the user in the "canonical" format is probably a better decision. However it _is_ a change and could be considered breaking. I require a +1 from @rwjblue and @Turbo87 to ship.

Closes #6599. Discarding this also should close #6599.

gfvcastro/ember-cli,balinterdi/ember-cli,ef4/ember-cli,cibernox/ember-cli,kellyselden/ember-cli,seawatts/ember-cli,kellyselden/ember-cli,pzuraq/ember-cli,kriswill/ember-cli,Turbo87/ember-cli,scalus/ember-cli,HeroicEric/ember-cli,patocallaghan/ember-cli,calderas/ember-cli,ef4/ember-cli,twokul/ember-cli,ef4/ember-cli,cibernox/ember-cli,romulomachado/ember-cli,mike-north/ember-cli,ember-cli/ember-cli,fpauser/ember-cli,HeroicEric/ember-cli,pzuraq/ember-cli,thoov/ember-cli,rtablada/ember-cli,kellyselden/ember-cli,sivakumar-kailasam/ember-cli,thoov/ember-cli,kriswill/ember-cli,jrjohnson/ember-cli,kanongil/ember-cli,jgwhite/ember-cli,mike-north/ember-cli,patocallaghan/ember-cli,calderas/ember-cli,romulomachado/ember-cli,trentmwillis/ember-cli,pzuraq/ember-cli,twokul/ember-cli,kategengler/ember-cli,fpauser/ember-cli,kriswill/ember-cli,sivakumar-kailasam/ember-cli,scalus/ember-cli,rtablada/ember-cli,raycohen/ember-cli,seawatts/ember-cli,trentmwillis/ember-cli,ember-cli/ember-cli,kategengler/ember-cli,kanongil/ember-cli,akatov/ember-cli,twokul/ember-cli,mike-north/ember-cli,calderas/ember-cli,buschtoens/ember-cli,scalus/ember-cli,kriswill/ember-cli,gfvcastro/ember-cli,kanongil/ember-cli,kellyselden/ember-cli,raycohen/ember-cli,patocallaghan/ember-cli,sivakumar-kailasam/ember-cli,Turbo87/ember-cli,jgwhite/ember-cli,balinterdi/ember-cli,akatov/ember-cli,calderas/ember-cli,Turbo87/ember-cli,fpauser/ember-cli,HeroicEric/ember-cli,thoov/ember-cli,gfvcastro/ember-cli,rtablada/ember-cli,romulomachado/ember-cli,trentmwillis/ember-cli,ef4/ember-cli,sivakumar-kailasam/ember-cli,scalus/ember-cli,asakusuma/ember-cli,gfvcastro/ember-cli,akatov/ember-cli,elwayman02/ember-cli,romulomachado/ember-cli,rtablada/ember-cli,seawatts/ember-cli,buschtoens/ember-cli,jgwhite/ember-cli,mike-north/ember-cli,akatov/ember-cli,thoov/ember-cli,elwayman02/ember-cli,pzuraq/ember-cli,jgwhite/ember-cli,twokul/ember-cli,jrjohnson/ember-cli,patocallaghan/ember-cli,HeroicEric/ember-cli,kanongil/ember-cli,trentmwillis/ember-cli,Turbo87/ember-cli,seawatts/ember-cli,asakusuma/ember-cli,cibernox/ember-cli,fpauser/ember-cli,ember-cli/ember-cli,cibernox/ember-cli