Return true if the key exists in redis

    CLI Example:

    .. code-block:: bash

        salt '*' redis.exists foo

def exists(key, host=None, port=None, db=None, password=None):
    '''
    Return true if the key exists in redis

    CLI Example:

    .. code-block:: bash

        salt '*' redis.exists foo
    '''
    server = _connect(host, port, db, password)
    return server.exists(key)