check_software: update glob to new buildout-shared signature files

Since slapos.recipe.cmmi version 0.18, the signature files no longer
match the glob pattern that was used here, so check that shared
parts do not reference software parts was not effective anymore.

slapos/testing/check_software.py

@@ -220,8 +220,13 @@ def checkSoftware(slap, software_url):
       ))
 
   # check this software is not referenced in any shared parts.
-  for signature_file in glob.glob(os.path.join(slap.shared_directory, '*', '*',
-                                               '.*slapos.*.signature')):
+  for signature_file in glob.glob(
+      os.path.join(
+          slap.shared_directory,
+          '*',
+          '*',
+          '.buildout-shared.json',
+      )):
     with open(signature_file) as f:
       signature_content = f.read()
     if software_hash in signature_content: