Allow end-users (or roles anyway) to reject default plugin repositories and features

Due to the design of chef attribute precedence specifically when dealing with hashes, there is no chef-way to enable a role to reject entirely the attributes set in the recipe files. One can change the values of the keys but one can't drop a key.

This change introduces an "excluding" array into the eclipse property set. The role can include a key from the plugins hash in the excluding array and then the recipe will reject any such keys from the plugins hash.

If there is a better chef or better ruby way to do this, please advise.