Add retries when creating boto3 client

[donnlee]

Fixes #12
This diff adds a retry loop when we attempt to create the boto3 client client.
Also creates a new amazon_rekognition config option boto_retries which is optional and defaults to 5 retries.

I'm currently using this patch in my home setup.
Here's the dev testing that was done:

• Tested with boto_retries: 0 (equivalent to how things worked before this diff).

With boto_retries set to 0, restarted HA until boto3.client() failed and exception was thrown. Output of Dev Tools > Logs is what we expect:

Error while setting up amazon_rekognition platform for image_processing
Traceback (most recent call last):
  File "/usr/src/homeassistant/homeassistant/helpers/entity_platform.py", line 178, in _async_setup_platform
    await asyncio.wait_for(asyncio.shield(task), SLOW_SETUP_MAX_WAIT)
  File "/usr/local/lib/python3.7/asyncio/tasks.py", line 442, in wait_for
    return fut.result()
  File "/usr/local/lib/python3.7/concurrent/futures/thread.py", line 57, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/config/custom_components/amazon_rekognition/image_processing.py", line 124, in setup_platform
    "the boto_retries setting. Retry counter was {}".format(retries))
Exception: Failed to create boto3 client. Maybe try increasing the boto_retries setting. Retry counter was 1

• Tested with boto_retries omitted from the amazon_rekognition config: Logging at level debug shows boto_retries is 5 (the default).

Restarted HA until a boto3.client() failure was encountered and retry was successful (boto_retries=5):

bash-5.0# cat home-assistant.log | grep rek
2020-05-02 16:14:32 WARNING (MainThread) [homeassistant.loader] You are using a custom integration for amazon_rekognition which has not been tested by Home Assistant. This
component might cause stability problems, be sure to disable it if you experience issues with Home Assistant.
2020-05-02 16:14:32 INFO (MainThread) [homeassistant.components.image_processing] Setting up image_processing.amazon_rekognition
2020-05-02 16:14:32 INFO (MainThread) [homeassistant.components.image_processing] Setting up image_processing.amazon_rekognition
2020-05-02 16:14:32 INFO (MainThread) [homeassistant.components.image_processing] Setting up image_processing.amazon_rekognition
2020-05-02 16:14:32 INFO (SyncWorker_13) [custom_components.amazon_rekognition.image_processing] boto3 client failed, retries=0
2020-05-02 16:14:32 INFO (SyncWorker_33) [custom_components.amazon_rekognition.image_processing] boto3 client failed, retries=0

Same test but with logging level set to debug. One camera was configured with boto_retries: 4, rest did not have a boto_retries yaml line:

bash-5.0# cat home-assistant.log | grep rek
2020-05-02 16:17:00 WARNING (MainThread) [homeassistant.loader] You are using a custom integration for amazon_rekognition which has not been tested by Home Assistant. This component might cause stability problems, be sure to disable it if you experience issues with Home Assistant.
2020-05-02 16:17:00 INFO (MainThread) [homeassistant.components.image_processing] Setting up image_processing.amazon_rekognition
2020-05-02 16:17:00 INFO (MainThread) [homeassistant.components.image_processing] Setting up image_processing.amazon_rekognition
2020-05-02 16:17:00 INFO (MainThread) [homeassistant.components.image_processing] Setting up image_processing.amazon_rekognition
2020-05-02 16:17:00 DEBUG (SyncWorker_0) [custom_components.amazon_rekognition.image_processing] boto_retries setting is 4
2020-05-02 16:17:00 DEBUG (SyncWorker_4) [custom_components.amazon_rekognition.image_processing] boto_retries setting is 5
2020-05-02 16:17:00 DEBUG (SyncWorker_33) [custom_components.amazon_rekognition.image_processing] boto_retries setting is 5
2020-05-02 16:17:00 INFO (SyncWorker_33) [custom_components.amazon_rekognition.image_processing] boto3 client failed, retries=0
bash-5.0#

[robmarkcole]

many thanks for this! Lets leave it undocument in the readme for now, as it probably should be for internal use by the integration but may be useful as a config option for debugging