Return a list of ec2 instances for the query.

def query_instances(session, client=None, **query):
    """Return a list of ec2 instances for the query.
    """
    if client is None:
        client = session.client('ec2')
    p = client.get_paginator('describe_instances')
    results = p.paginate(**query)
    return list(itertools.chain(
        *[r["Instances"] for r in itertools.chain(
            *[pp['Reservations'] for pp in results])]))