Allowing buildd-master to control 'binary-mangling' feature

In order to make the lp-buildd even more generic we could allow the 'binary-mangling' feature to be controlled by the build-master side.

This way it would easier to cope with the fact that some private PPA (like ubuntu-security) requires that feature and others don't.

It will obviously require changes in the launchpad code and model, but that's where the logic should be, IMHO.

We could revise the approach taken before, when we decided to pass purpose=PPA to the buildd in order to disable such feature.