fix(update_distfiles): Add --download option to for incremental updates

Temporary workaround until we have a version of emirrordist that can
operate directly on remote storage. Download the entire mirror (if it
isn't already local) so emirrordist can make an incremental update.

Not great but it will do for now.
fix(bootstrap_sdk): Rename final output tarball

The name "coreos-sdk-amd64-..." makes much more sense for general
distribution than "stage4..." so after catalyst is done rename the final
tarball and fixup the DIGESTS file to refer to the new name.
fix(update_distfiles): Add option to control gsutil's parallelism

gsutil can be hard to follow when parallel upload/downloads are enabled.
"I see it is transferring something, but what?" So this provides an
option to disable that for debugging purposes.
fix(update_distfiles): Set temporary directory

Provide an alternate directory for in-progress downloads to avoid
uploading any by accident.
fix(bootstrap_sdk): Use the latest SDK tarball as a fallback seed.

When running bootstrap_sdk for the first time on a host the default of
using a previous catalyst run as a seed won't work but this may be a
fresh SDK install so that tarball is probably around somewhere and will
work as a default seed for most things.
fix(bootstrap_sdk): Add option to upload built SDK and packages

This automates the process of uploading the fresh SDK release.
fix(enter_chroot): Use .boto from the user's HOME

We don't have any private overlays but users will likely have their own
credentials in ~/.boto so make it available in the chroot.
Merge pull request #8 from marineam/buildbot

Buildbot