Refactoring of the centos-ruby-* images to share code and avoid duplication

- Removed `run` from `centos-ruby-builder` if favor of the `centos-ruby` run command
- Backported `HOME` and `PATH` environment variables to `centos-ruby`
- Fixed `ruby_context` to use `exec` properly
- Removed nodejs010 SCL detection (it is installed in base image anyway...)
- Backported `bundle_exec` and `bundle_install` functions from builder image to base to address the `exec` problem
- Backported `RAILS_ENV` and `RACK_ENV` variables from the builder image
- Docker preserve the perms of the files when ADD, so the chmod +x layer is not necessary.
- Many more minor tweaks

After this change, the `centos-ruby-builder` will consist just from the `start` command which overrides itself. No more code duplication ;-)