checksum: Refactor creating checksum files

Instead of iterating over the images metadata and appending the checksum
to relevant files immediately, we should store them and write only once.

This avoid an issue when the same image is mentioned in the metadata
multiple times. This happens for source images that are listed under
each binary arch.

The unified isos script is updated to use the exact same logic and code.
This also uncovered a problem with the metadata for debuginfo unified
isos: their paths in metadata were incorrect, which lead to missing
checksums.

Fixes: https://pagure.io/pungi/issue/667
Fixes: https://pagure.io/pungi/issue/668
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>

tests/helpers.py

@@ -94,7 +94,7 @@ class DummyCompose(object):
         self.log_debug = mock.Mock()
         self.log_warning = mock.Mock()
         self.get_image_name = mock.Mock(return_value='image-name')
-        self.image = mock.Mock(path='Client/i386/iso/image.iso', can_fail=False)
+        self.image = mock.Mock(path='Client/i386/iso/image.iso', can_fail=False, size=123)
         self.im = mock.Mock(images={'Client': {'amd64': [self.image]}})
         self.old_composes = []
         self.config_dir = '/home/releng/config'