UI Doesn't support custom S3 endpoints for backups

[dmayle]

The UI uses the python boto3 library, which doesn't respect the AWS_ENDPOINT environment variable ( boto/boto3#1375 ).

Two updates need to be made, at:
https://github.com/zalando/postgres-operator/blob/master/ui/operator_ui/spiloutils.py#L269
https://github.com/zalando/postgres-operator/blob/master/ui/operator_ui/spiloutils.py#L290

client('s3', endpoint_url=environ.get('AWS_ENDPOINT')).list_objects(