Make "contributors' guide" link relative

 * per https://github.com/blog/1395-relative-links-in-markup-files
Use UploadAdapter in Build.

Remove Pipeline from Build.
Reducing public interface on Pipeline
Inline s3_upload to reduce public interface on Pipeline
#upload_r moved from BulkUploader to Pipeline
Removed BulkUploader; refactored functionality into Pipeline
Moved #create to private on Pipeline
Removing unused stemcell path methods
vSphere cpi tests use Build.candidate to download stemcell
Removing unused download_stemcell from Pipeline
Delegate from Pipeline#publish_stemcell to Build#upload_stemcell
Remove unused #cleanup_stemcells
Remove unused private #stemcell_filename
Delegating from Pipeline#gems_dir_url to Build#gems_dir_url
Inlining Pipeline#create private method
Revert "Revert "Boostrap no longer creates an RDS database for a public mysql offering.""

This reverts commit 952bb0c6d3118e1231689f90e2b87048ee83bf22.