Model callback enhancement

Allows cfg, cmd, pre, post and expect blocks to reset the callback array or change from push to unshift.

Use case is in custom models which monkey-patch the distribution model. You might want to replace for example distribution ```cmd :secret``` with your own version, like such:

cmd :secret clear: true do
  ...
end

Or if you instead want to unshift your block to be first to be called:

cfg :ssh do prepend: true do
  ..
end

Closes #1384 

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