Confusing buildSmells configuration

[martinlau]

<buildSmells> is documented as the smells to include, however the code in TileValidator.validateModel complains if the smell is NOT included in the configured build smells. For example:

if (model.repositories && !buildSmells.contains(SMELL_REPOSITORIES)) {
    log.error("Tile follows bad practice and has repositories section. Please use settings.xml.")
    validModel = null
}

That is, if the model contains repositories, and buildSmells does NOT contain "repositories", then complain.

As I see it there are a couple of non-ideal options:

1. update documentation on how to use <buildSmells> (however <buildSmells>xxx</buildSmells> being used to exclude xxx violates the principal of least surprise IMHO)
2. flip the logic on <buildSmells> to make this the definitive list of what to use, rather than what to exclude (however this breaks existing users)
3. mark <buildSmells> as deprecated and introduce <excludeBuildSmells> to make the existing logic explicit (this also opens the door for <includeBuildSmells> which means if you add a new smell I don't need to go through and manually exclude it everywhere I disagree with it - it's a opt-in model rather than opt-out).

Discuss, and a PR shall be yours.