Remove public stemcell publishing rake tasks

[finishes #54861786]

This code is tightly coupled to the Atmos blobstore that we no longer
use (we migrated to S3). We'll need this functionality again in the future
so this is being removed in one commit so that we can easily revert or
reference it later. We're currently extracting all of the code from the
Rake tasks into Ruby classes and we don't want to port all of this code
if we're going to need to change most of it soon.
we do not need the old style path anymore
remove unused variables
add the default disk size as an attribute of an infrastructure
begin to move stemcell.rake into ruby classes starting with default_options
Rip the bag of methods from the rake task out into a bag of methods class
style guide fixes
add missing require to rake task
move refactored #default_options method into the rake method bag
remove unused git dirtiness checking methods
test monolithic #build method
explain openstack system disk size difference to other stemcells
test that user can specify stemcell work directory
fix a botched rubymine rename refactor