image-build: add arch name(s) in image config file name
    
    Pungi write image config file with name of <format>-<name>.cfg, if there
    are two or more image configs present for different arches under the same
    variant and with same format & name, the config file can be overwritten,
    and result in invalid image conf file.
    
    Example:
    
    image_build = {
        '^Server$': [
            {
                'image-build': {
                    'format': [('qcow2', 'qcow2'),],
                    'name': 'fedora-guest-image',
                    'target': 'guest-fedora-26-image',
                    'version': '26',
                    'ksurl': "git://git.example.com/ks.git?fedora#HEAD",
                    'kickstart': "fedora-26-kvm.ks",
                    'ksversion': 'f26',
                    'distro': 'fedora-26',
                    'disk-size': '10',
                    'arches': ['x86_64'],
                    'repo': ["http://example.com/linux/fedora/26/Everything/x86_64/os", ]
                }
            },
            {
               'image-build': {
                    'format': [('qcow2', 'qcow2'),],
                    'name': 'fedora-guest-image',
                    'target': 'guest-fedora-26-image',
                    'version': '26',
                    'ksurl': "git://git.example.com/ks.git?fedora#HEAD",
                    'kickstart': "fedora-26-kvm.ks",
                    'ksversion': 'f26',
                    'distro': 'fedora-26',
                    'disk-size': '10',
                    'arches': ['ppc64le'],
                }
            },
        ],
    }
    
    In this case, config file "qcow2_guest-fedora-26-image.cfg" will be
    created for both x86_64 and ppc64le under the same variant dir, and
    there is a high chance it will be over-written while Pungi creating the
    koji task. We can add arch name(s) in config filename to avoid that.
    
    Signed-off-by: Qixiang Wan <qwan@redhat.com>