cros_test_lib: StackedSetup: clean up code a bit
cros_test_lib: StackedSetup: clean up code a bit

The StackedSetup class has a few helper functions that aren't used by
anything else.  Move them into the StackedSetup class.  This makes it
more obvious that the funcs "belong" to this object.  The namespace
stays clean as StackedSetup is used as a metaclass -- that means it
generates a new class on the fly rather than itself being inherited.

While we're here, also add a few more docstrings to help the reader.

BUG=None
TEST=`./buildbot/run_tests` passes

Change-Id: I0ce2a1a96dbf50c45d66373040a6e921700827dd
Reviewed-on: https://chromium-review.googlesource.com/175435
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Commit-Queue: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite