integration-tests: Increase MQTT keep alive interval

With latest FreeRTOS TCP/IP stack, due to a timing issue, most of the
MQTT tests fail on FVP, because of connection reset. Therefore,
increase the MQTT keep alive timeout `MQTT_KEEP_ALIVE_INTERVAL_SECONDS`
from `60` to `300` seconds.

Signed-off-by: Devaraj Ranganna <devaraj.ranganna@arm.com>