Replaces direct use of aiokafka with osm_common message bus in agent and

Replaces direct use of aiokafka with osm_common message bus in agent and
lcmclient

Changes config handling to comply with the way it is handled in other modules,
by using a config file and overriding it with env vars.

Adds unit tests for message_bus_client.

Mon client remains using aiokafka directly, as there is no support yet for
auto_offset_reset configuration in osm_common.

Change-Id: I99615287cc934ce310105e86544a6bfe26bc0673
Signed-off-by: Benjamin Diaz <bdiaz@whitestack.com>