pop_arg "default" to default to None
pop_arg "default" to default to None

- Changed the "default" arg for the pop_arg() function to default to
  None.

- Also, for source_to_object() function, altered the test for
  detecting special-case of collections.OrderedDict from
  startswith("[") to startswith("[(").  By making the test more
  strict, we allow for simple lists to be processed as lists rather
  than as collections.OrderedDict.

Change-Id: I4d861307653332530023d53a89d191e828aaed5c
Signed-off-by: Michael Walsh <10404802c7de463f402c4a18638106ca1b12ef61@us.ibm.com>

openbmc/openbmc-test-automation,openbmc/openbmc-test-automation