Added default topic arn to master config. Added tests to ensure pipeline

[p5k6]

defs override default config.

Reference issue #240

One note - the DEFAULT_TOPIC_ARN in dataduct/etl/etl_pipeline.py is defined within the class, rather than outside (as other vars are - such as S3_ETL_BUCKET), because

• it keeps the global namespace a little cleaner
• mocking is far easier, as the var is local to the class level at that point

[coveralls]

Coverage increased (+0.08%) to 64.343% when pulling 8c1584e on p5k6:add_topic_arn_config into 05b81ea on coursera:develop.