'Constructor for Storage.
Args:
model: db.Model, model class
key_name: string, key name for the entity that has the credentials
key_value: string, key value for the entity that has the credentials
property_name: string, name of the property that is an CredentialsProperty'

def __init__(self, model_class, key_name, key_value, property_name):

    self.model_class = model_class
    self.key_name = key_name
    self.key_value = key_value
    self.property_name = property_name
