Allows to set the account meta key by setting hp_account_meta_key, needed to generate temp urls using the HP provider, explicitly, instead of using hp_secret_key. If hp_account_meta_key is not given hp_secret_key is used as hp_account_meta_key.
HP uses a different strategy to create the signature that is passed to swift than OpenStack.  As the HP provider is broadly used by OpenStack users the OpenStack strategy is applied when the @hp_account_meta_key is given.