update_engine: brillo_update_payload: Switch to major version 2

Per this CL, we switch the payloads to major version 2 from servers.  For more
information regarding this switch refer to the attached issue. We also change
the default major version to 2.

BUG=chromium:794404
TEST=brillo_update_payload

Change-Id: I8403805c07210c3d947100efce74dc184a022cdb
Reviewed-on: https://chromium-review.googlesource.com/1234590


Commit-Ready: Amin Hassani <ahassani@chromium.org>
Tested-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Don Garrett <dgarrett@chromium.org>
Reviewed-by: Sen Jiang <senj@chromium.org>