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/multi-compose.conf

@@ -0,0 +1,19 @@
+[general]
+release_name = Multi Compose
+release_short = multi
+release_version = 1.0
+release_type = ga
+compose_type = nightly
+target = ../_composes/
+extra_args = --quiet
+
+[server]
+config = server.conf
+
+[client]
+config = client.conf
+
+[resilient-storage]
+config = resilient-storage.conf
+depends_on = server
+failable = yes