Add Asset Validator to default repo configuration

The Asset and Image Validator plugin allows to validate uploader images or files, rejecting those that don't match a certain set of conditions (size, extension, resolution, etc...) The Image Validator configuration nodes are part of the plugin bootstrap content but the generic Asset doesn't, so we should add that.

Also, the documentation on http://www.onehippo.org/7_7/library/concepts/editor-interface/image-and-asset-upload-validation.html could be tweaked a bit and a variant for 7_6 should also be added.