Extract container converters

“objects.ConfigObject” contains two useful functions for working with
containers of serialized objects, “_ContainerToDicts” and
“_ContainerFromDicts”. This patch separates those functions and moves
them into “objectutils” as they'll be useful for converting parts of the
QA configuration to objects. Unittests are added to cover the parts not
already tested through other code paths.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>