Move crowbar_node and crowbar_data out of ChefObject

These methods are specific to NodeObject and ProposalObject, so they
should not be inherited by other classes.

They were used only on two other places, so move them over to where
they're called.
Merge pull request #1040 from bkutil/move_chef_loaders_out

Move crowbar_node and crowbar_data out of ChefObject