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>

tests/fixtures/DP-1.0-20161013.t.4/compose/Client/x86_64/os/.treeinfo

@@ -0,0 +1,33 @@
+[general]
+; WARNING.0 = This section provides compatibility with pre-productmd treeinfos.
+; WARNING.1 = Read productmd documentation for details about new format.
+arch = x86_64
+family = Dummy Product
+name = Dummy Product 1.0
+platforms = x86_64
+timestamp = 1476343975
+variant = Client
+variants = Client
+version = 1.0
+
+[header]
+type = productmd.treeinfo
+version = 1.2
+
+[release]
+name = Dummy Product
+short = DP
+version = 1.0
+
+[tree]
+arch = x86_64
+build_timestamp = 1476343975
+platforms = x86_64
+variants = Client
+
+[variant-Client]
+id = Client
+name = Client
+type = variant
+uid = Client
+