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/fixtures/basic-metadata/compose/metadata/composeinfo.json

@@ -0,0 +1,25 @@
+{
+    "header": {
+        "type": "productmd.composeinfo",
+        "version": "1.2"
+    },
+    "payload": {
+        "compose": {
+            "date": "20181001",
+            "id": "Mixed-1.0-20181001.n.0",
+            "respin": 0,
+            "type": "nightly"
+        },
+        "release": {
+            "internal": false,
+            "name": "Dummy Product",
+            "short": "Mixed",
+            "type": "ga",
+            "version": "1.0"
+        },
+        "variants": {
+            "Var0": {
+            }
+        }
+    }
+}

tests/fixtures/basic-metadata/compose/metadata/images.json

@@ -0,0 +1,18 @@
+{
+    "header": {
+        "type": "productmd.images",
+        "version": "1.2"
+    },
+    "payload": {
+        "compose": {
+            "date": "20181001",
+            "id": "Mixed-1.0-20181001.n.0",
+            "respin": 0,
+            "type": "nightly"
+        },
+        "images": {
+            "Var0": {
+            }
+        }
+    }
+}

tests/fixtures/basic-metadata/compose/metadata/modules.json

@@ -0,0 +1,18 @@
+{
+    "header": {
+        "type": "productmd.modules",
+        "version": "1.2"
+    },
+    "payload": {
+        "compose": {
+            "date": "20181001",
+            "id": "Mixed-1.0-20181001.n.0",
+            "respin": 0,
+            "type": "nightly"
+        },
+        "modules": {
+            "Var0": {
+            }
+        }
+    }
+}

tests/fixtures/basic-metadata/compose/metadata/osbs.json

@@ -0,0 +1,4 @@
+{
+    "Var0": {
+    }
+}

tests/fixtures/basic-metadata/compose/metadata/rpms.json

@@ -0,0 +1,18 @@
+{
+    "header": {
+        "type": "productmd.rpms",
+        "version": "1.2"
+    },
+    "payload": {
+        "compose": {
+            "date": "20181001",
+            "id": "Mixed-1.0-20181001.n.0",
+            "respin": 0,
+            "type": "nightly"
+        },
+        "rpms": {
+            "Var0": {
+            }
+        }
+    }
+}