cbuildbot_stages.py: Refactoring of ReportStage.

This refactoring is a step toward removing the requirement of an
ArchiveStage in order to do anything in the ReportStage.  It reduces the
usage of the ArchiveStage object during ReportStage.PerformStage() to
one place.  A subsequent CL will complete the purge.  This will allow
runs without an ArchiveStage (or board) to still do something in

TEST=`cbuildbot --remote --buildbot --debug x86-mario-paladin`

Change-Id: I2467c659ac2b0dd32635d9a321938cc69296a946
Reviewed-on: https://chromium-review.googlesource.com/182228
Tested-by: Matt Tennant <mtennant@chromium.org>
Reviewed-by: David James <davidjames@chromium.org>
Commit-Queue: Matt Tennant <mtennant@chromium.org>
7 files changed