createiso: Allow reusing old images

This patch allows Pungi to reuse ISO image created in previous compose
if a list of assumptions proves to hold:

 * If image is bootable, buildinstall phase must have been reused too.
 * Compose configuration must have not changed (except for a few
   whitelisted options).
 * Volume ID of the ISO much not have changed.
 * No RPM on the ISO must have changed.

The ISO also contains other files. Changes in extra files and product ID
certificates should be visible in configuration (the SHA will differ).
Similarly any repodata configuration would be reflected in
configuration.

JIRA: RHELCMP-5969
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>

pungi/util.py

@@ -1130,3 +1130,9 @@ class PartialFuncThreadPool(ThreadPool):
     @property
     def results(self):
         return self._results
+
+
+def read_json_file(file_path):
+    """A helper function to read a JSON file."""
+    with open(file_path) as f:
+        return json.load(f)