Manifest manager should not create cantaloupe directory

[chiatt]

The manifest manager tries to create a directory for uploaded cantaloupe files:

       if not os.path.exists(settings.CANTALOUPE_DIR):
            os.mkdir(settings.CANTALOUPE_DIR)

If Apache does not have permissions to write that directory, the manifest manager will fail to save a manifest. This is a problem if the system is configured to use s3 and doesn't need a directory written to the application server. We should therefore remove this and let the administrator create a directory for Cantaloupe uploads themselves.