Add script to orchestrate multiple composes

It may make sense to break a big compose into smaller chunks that can be
done independently. This script allows describing the smaller parts,
runs them with correct dependencies and arranges the result to look like
a single big compose.

All parts use the same koji event, that is either obtained from Koji, or
from command line argument.

JIRA: COMPOSE-2654
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>

tests/data/server.conf

@@ -0,0 +1,16 @@
+from dummy-pungi import *
+
+tree_variants = ["Server"]
+variants_file = "multi-compose-variants.xml"
+pkgset_repos = {
+    "i386": [
+        "{{configdir}}/repo",
+    ],
+    "x86_64": [
+        "{{configdir}}/repo",
+    ],
+    "s390x": [
+        "{{configdir}}/repo",
+    ],
+}
+extra_isos = {}