Fix crashing generator called from celery task

In some cases of celery configuration task can receive broken PATH variable from user env and subprocess.call(['gzip', '-f', os.path.join(conf.ROOT_DIR, filename)]) crash with OSError(2, 'No such file or directory').
I add some code with additional settings variables to gzip sitemaps part of generator: using python gzip standard library and modify gzip sitemaps by gzip code to use path to gzip binary.