Subscribe to a Topic.

    CLI example to delete a topic::

        salt myminion boto_sns.subscribe mytopic https https://www.example.com/sns-endpoint region=us-east-1

def subscribe(topic, protocol, endpoint, region=None, key=None, keyid=None, profile=None):
    '''
    Subscribe to a Topic.

    CLI example to delete a topic::

        salt myminion boto_sns.subscribe mytopic https https://www.example.com/sns-endpoint region=us-east-1
    '''
    conn = _get_conn(region=region, key=key, keyid=keyid, profile=profile)
    conn.subscribe(get_arn(topic, region, key, keyid, profile), protocol, endpoint)
    log.info('Subscribe %s %s to %s topic', protocol, endpoint, topic)
    try:
        del __context__[_subscriptions_cache_key(topic)]
    except KeyError:
        pass
    return True