[DX] Re-work generate:bundle for simple app bundles #290

@weaverryan please review my pull request ;)
Below this, in the namespace area, I think we should change the messaging based on the value of `$shared` (I mentioned this already here: https://github.com/sensiolabs/SensioGeneratorBundle/pull/299/files#r16630719).

Also, we need the `Validators:: validateBundleNamespace` to *not* enforce the vendor namespace if `$shared=false`.