Get all instances matching a tag.

def _aws_get_instance_by_tag(region, name, tag, raw):
    """Get all instances matching a tag."""
    client = boto3.session.Session().client('ec2', region)
    matching_reservations = client.describe_instances(Filters=[{'Name': tag, 'Values': [name]}]).get('Reservations', [])
    instances = []
    [[instances.append(_aws_instance_from_dict(region, instance, raw))  # pylint: disable=expression-not-assigned
      for instance in reservation.get('Instances')] for reservation in matching_reservations if reservation]
    return instances