'Do the actual credential deletion work (default implementation).
:param match_fn: function that takes a credential dict as the
parameter and returns true or false if the
identifier matches the credential dict.'

def _delete_credentials(self, match_fn):

    for cr in self.list_credentials():
        if match_fn(cr):
            try:
                self.credential_api.delete_credential(cr['id'])
            except exception.CredentialNotFound:
                LOG.debug('Deletion    of    credential    is    not    required:    %s', cr['id'])
