AWS system test example_redshift: increase timeout

[vincbeck]

Some of our recent system test execution failed because a sensor timed out waiting the cluster to be available. See errors below:

INFO     airflow.task:taskinstance.py:1358 Executing <Task(RedshiftClusterSensor): wait_cluster_available_after_resume> on 2021-01-01T00:00:00+00:00
INFO     airflow.task.operators:redshift_cluster.py:64 Poked cluster env123917c5-redshift-cluster for status 'available', found status 'resuming'
ERROR    airflow.task:taskinstance.py:1903 Task failed with exception
--
Traceback (most recent call last):
File "/opt/airflow/airflow/sensors/base.py", line 242, in execute
raise AirflowSensorTimeout(message)
airflow.exceptions.AirflowSensorTimeout: Sensor has timed out; run duration of 905.869984 seconds exceeds the specified timeout of 900.

Let's increase the timeouts avoid this kind of timeouts

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