createrepo: Add extra modulemd files to the repo

This is a workaround for modularity design issues and DNF bugs. If there
were gaps in contexts, DNF has trouble handling the upgrades. Thus we
may need to add module metadata for older versions of previously
released module streams and add the missing contexts.

JIRA: RHELCMP-982
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>

tests/fixtures/fake-modulemd.yaml

@@ -0,0 +1,20 @@
+---
+document: modulemd
+version: 2
+data:
+  name: mymodule
+  stream: master
+  version: 1
+  context: cafe
+  arch: x86_64
+  summary: Dummy module
+  description: Dummy module
+  license:
+    module:
+    - Beerware
+    content:
+    - Beerware
+  artifacts:
+    rpms:
+      - foobar-0:1.0-1.noarch
+...