Make the logging group, and caching groups easier to get at.

This simplifies a bit of a hack that was in cbuildbot, and is useful
for downstream consumers of this class.

BUG=chromium-os:32910
TEST=invocation of cbuildbot/cros_sdk --help

Change-Id: I816ec4e51a8c3ba912ce298f5612d30a6e2ed43f
Reviewed-on: https://gerrit.chromium.org/gerrit/35178
Reviewed-by: Brian Harring <ferringb@chromium.org>
Tested-by: Brian Harring <ferringb@chromium.org>
Commit-Ready: Brian Harring <ferringb@chromium.org>
diff --git a/scripts/cbuildbot.py b/scripts/cbuildbot.py
index 29c355e..1a7b7e9 100644
--- a/scripts/cbuildbot.py
+++ b/scripts/cbuildbot.py
@@ -944,10 +944,9 @@
   #
   # Debug options
   #
-  group = parser.get_option_group('--debug')
-
   # Temporary hack; in place till --dry-run replaces --debug.
   # pylint: disable=W0212
+  group = parser.debug_group
   debug = [x for x in group.option_list if x._long_opts == ['--debug']][0]
   debug.help += "  Currently functions as --dry-run in addition."
   debug.pass_through = True