Exit 1 when container image prepare fails

Currently if an exception is thrown during the container image prepare,
we log it but we still return 0. Anything that is running the command
doesn't know it had issues.

Change-Id: I87be8c3c7268399d86856eb47e801258d117a172
Closes-Bug: #1835837
(cherry picked from commit 4596f0f332)

scripts/tripleo-container-image-prepare

@@ -134,3 +134,4 @@ if __name__ == '__main__':
         print(result)
     except Exception as e:
         log.exception("Image prepare failed: {}".format(e))
+        sys.exit(1)