Gets the name of the first existing user cert for a given user and host.

        Args:
            user (str): The name of the user.
            host (str): The name of the host.

        Examples:
            Get the name for the "myuser" user cert at "cool.vertex.link":

                usercertname = cdir.getUserForHost('myuser', 'cool.vertex.link')

        Returns:
            str: The cert name, if exists.

def getUserForHost(self, user, host):
        '''
        Gets the name of the first existing user cert for a given user and host.

        Args:
            user (str): The name of the user.
            host (str): The name of the host.

        Examples:
            Get the name for the "myuser" user cert at "cool.vertex.link":

                usercertname = cdir.getUserForHost('myuser', 'cool.vertex.link')

        Returns:
            str: The cert name, if exists.
        '''
        for name in iterFqdnUp(host):
            usercert = '%s@%s' % (user, name)
            if self.isUserCert(usercert):
                return usercert