cbuildbot: Don't create a DistributedBuilder if manifest_version=False

Simplify IsDistributedBuilder() implementation and use the value of
manifest_version config parameters instead of relying on build_type.

BUG=chromium:339599
TEST=trybot, unittest

Change-Id: Iefc1676fe38ad7ad630c7bab6e5564af462cd2a2
Reviewed-on: https://chromium-review.googlesource.com/184497
Reviewed-by: David James <davidjames@chromium.org>
Commit-Queue: Gaurav Shah <gauravsh@chromium.org>
Tested-by: Gaurav Shah <gauravsh@chromium.org>
2 files changed