Switch to another user (admin only).

        This API allows remote admin/service accounts
        to impersonate a user.  Used mostly by services
        that manage their own authentication/sessions.

def setCellUser(self, iden):
        '''
        Switch to another user (admin only).

        This API allows remote admin/service accounts
        to impersonate a user.  Used mostly by services
        that manage their own authentication/sessions.
        '''
        if not self.user.admin:
            mesg = 'setCellUser() caller must be admin.'
            raise s_exc.AuthDeny(mesg=mesg)

        user = self.cell.auth.user(iden)
        if user is None:
            raise s_exc.NoSuchUser(iden=iden)

        self.user = user
        return True