code: def token_cache_pkgs(source=None, release=None):
      
      packages = []
      if enable_memcache(source=source, release=release):
          packages.extend(['memcached', 'python-memcache'])
      return packages
docs: Determine additional packages needed for token caching
  
      @param source: source string for charm
      @param release: release of OpenStack currently deployed
      @returns List of package to enable token caching