config_lib: Move OverrideConfigForTrybot to config_lib.

This Override method is now site independent. Signify this by moving
it to config_lib.

Also create some site independent unittests for it.

CQ-DEPEND=CL:282072
BUG=chromium:457439
TEST=lint + run_tests

Change-Id: Ifa66fac798fb991397324a40e712e0a2d8b68044
Reviewed-on: https://chromium-review.googlesource.com/282053
Reviewed-by: Don Garrett <dgarrett@chromium.org>
Tested-by: Don Garrett <dgarrett@chromium.org>
Commit-Queue: Don Garrett <dgarrett@chromium.org>
diff --git a/scripts/cbuildbot.py b/scripts/cbuildbot.py
index 243aece..1a0a063 100644
--- a/scripts/cbuildbot.py
+++ b/scripts/cbuildbot.py
@@ -23,7 +23,6 @@
 from chromite.cbuildbot import cbuildbot_run
 from chromite.cbuildbot import config_lib
 from chromite.cbuildbot import constants
-from chromite.cbuildbot import chromeos_config
 from chromite.cbuildbot import manifest_version
 from chromite.cbuildbot import remote_try
 from chromite.cbuildbot import repository
@@ -1187,7 +1186,7 @@
     stack.Add(critical_section.ForkWatchdog)
 
     if not options.buildbot:
-      build_config = chromeos_config.OverrideConfigForTrybot(
+      build_config = config_lib.OverrideConfigForTrybot(
           build_config, options)
 
     if options.mock_tree_status is not None: