samples: net: mqtt_publisher: Add exit code and Kconfig options

Modify the mqtt_publisher sample app to return an exit code after
its tests have been run. Add Kconfig options to set the number of
test iterations per TCP connection as well as the number of TCP
connections to make to the server while keeping the default values
intact. Further add a config overlay file to lower the number of
TCP connections and test iterations used.

Signed-off-by: Patrik Flykt <patrik.flykt@intel.com>