integration-test: Update TCP keep alive interval config

With latest FreeRTOS TCP/IP stack, due to a timing issue, most of the
MQTT tests fail on FVP, because of TCP keep alive timeout. Therefore,
increase the TCP keep alive timeout `ipconfigTCP_KEEP_ALIVE_INTERVAL`
from `20` to `300` seconds.

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