Get an active session in the target account.

def get_session(self, account_id):
        """Get an active session in the target account."""
        if account_id not in self.account_sessions:
            if account_id not in self.config['accounts']:
                raise AccountNotFound("account:%s is unknown" % account_id)

            self.account_sessions[account_id] = s = assumed_session(
                self.config['accounts'][account_id]['role'], "Sphere11")
            s._session.user_agent_name = "Sphere11"
            s._session.user_agent_version = "0.07"
        return self.account_sessions[account_id]