def get_ubuntu_distribution_version():
    """
    Discover and return Ubuntu distribution version.

    :return: the Ubuntu distribution version of the build server.
    example: xenial, bionic
    """
    import distro
    distribution = distro.linux_distribution()
    if distribution[0] == 'Ubuntu' and distribution[1] == '16.04':
        return 'xenial'
    elif distribution[0] == 'Ubuntu' and distribution[1] == '18.04':
        return 'bionic'
    else:
        raise ValueError('Unsupported distribution', distribution)
