samples: net: mqtt_publisher: Convert to new logging system
samples: net: mqtt_publisher: Convert to new logging system

Use new logging system with support for network log level.

Signed-off-by: Jukka Rissanen <f9f077d1da4c7aa947f9864fa5a28c3588f6a45d@linux.intel.com>

punitvara/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,Vudentz/zephyr,nashif/zephyr,ldts/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,finikorg/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,punitvara/zephyr,Vudentz/zephyr,galak/zephyr,explora26/zephyr,nashif/zephyr,explora26/zephyr,ldts/zephyr,explora26/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,Vudentz/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,explora26/zephyr,Vudentz/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,ldts/zephyr,galak/zephyr,galak/zephyr,ldts/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr