net/mqtt: Add the MQTT Publisher sample application

This commit adds the MQTT Publisher sample application for Zephyr.

The following MQTT API routines are exercised in this sample:

- mqtt_init
- mqtt_tx_connect
- mqtt_tx_pingreq
- mqtt_tx_publish
- mqtt_tx_disconnect

The following MQTT API data structures are used in this application:

- struct mqtt_ctx
- struct mqtt_connect_msg
- struct mqtt_publish_msg

This sample application exemplifies how to send MQTT PUBLISH messages
with different QoS values.

A README file is also included in this patch.

Change-Id: I2b41b276d5178a2cd1b07b031c38bff481885fff
Signed-off-by: Flavio Santes <flavio.santes@intel.com>