Dynamically rebind 'after rubber:install_packages' callbacks to 'after rubber:setup_volumes' during bootstrap to avoid duplicating work if an attached volume would contain the deployed application.

This is one of the hackiest things I've done in a while.  But it helped drop a create_staging task I had from 1h 25m to 47m.  @wr0ngway and @davebenvenuti, mind reviewing?