Fetches a list all security groups.

        :returns: List of SecurityGroup objects

def list(self, **params):
        """Fetches a list all security groups.

        :returns: List of SecurityGroup objects
        """
        # This is to ensure tenant_id key is not populated
        # if tenant_id=None is specified.
        tenant_id = params.pop('tenant_id', self.request.user.tenant_id)
        if tenant_id:
            params['tenant_id'] = tenant_id
        return self._list(**params)