Add script to generate unified ISOs

This a standalone script that will look into a compose and create
unified ISO for each architecture. The ISO contains RPM repositories for
all variants that have the arch.

Known issues:
 * The filename does not respect settings. This is tricky because the
   name could include variant name, which we don't have here (by design
   of unified ISO).
 * The same is true for volume id.

In order to test the feature without running actual compose, we need to
add essentially a big chunk of compose. Most of the files are empty, as
their content is never accessed.

Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>

MANIFEST.in

@@ -10,4 +10,4 @@ include doc/*
 include tests/*
 include tests/data/*
 include tests/data/*/*
-recursive-include tests/fixtures *.json *.xml *.bz2 *.gz
+recursive-include tests/fixtures *.json *.xml *.bz2 *.gz *.iso *.log MD5SUM SHA1SUM SHA256SUM treeinfo