[BAHIR-24] fix MQTT Python code, examples, add tests

[BAHIR-24] fix MQTT Python code, examples, add tests

Changes in this PR:

- remove unnecessary files from streaming-mqtt/python
- updated all *.py files with respect to the modified
  project structure pyspark.streaming.mqtt --> mqtt
- add test cases that were left out from the import and
  add shell script to run them:
    - streaming-mqtt/python-tests/run-python-tests.sh
    - streaming-mqtt/python-tests/tests.py
- modify MQTTTestUtils.scala to limit the required disk storage space
- modify bin/run-example script to setup PYTHONPATH to run Python examples

Closes #10