Adds OSMMON_KAFKA_LOG_LEVEL env var

Also removes unnecessary code from producer.py and moves
the consumer in common_consumer inside run method to ease
mocking during tests.

Signed-off-by: Benjamin Diaz <bdiaz@whitestack.com>
Change-Id: Ib7e0b9423a7c7d2acdd12f1bcb75b20eb3a7ea72