script to generate the sitemap.xml for docs.openstack.org

This script crawls all available sites on http://docs.openstack.org and extracts
all URLs. Based on the URLs the script generates a sitemap for search engines
according to the protocol described at http://www.sitemaps.org/protocol.html.

Change-Id: Id7839d2048989da503d31e436455aea9bb4cdc1d
Merge "script to generate the sitemap.xml for docs.openstack.org"