samples/net/mqtt: Reduce tx/rx buffer size and update mqtt_publish_read

Set CONFIG_IP_BUF_RX_SIZE and CONFIG_IP_BUF_TX_SIZE to 4.
Modify mqtt_publish_read to retrieve topic name and message as strings.
Change printf for printk.

Change-Id: I6746846cb3359096d87dfbb3134d6a3a95bf087e
Signed-off-by: Flavio Santes <flavio.santes@intel.com>