Use build data as as pubdate
Use build data as as pubdate

The guide used a hardcode date which is not updated at all, let's follow
the usual convention to add the build date here.

Change-Id: Ifde1a1d17a4930474d1de7d88cd1ca17485e5b72
Closes-Bug: #1211638

kairoaraujo/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,Akasurde/openstack-manuals,openstack/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,gongysh2004/openstack-manual,openstack/openstack-manuals,gongysh2004/openstack-manual,Akasurde/openstack-manuals,kairoaraujo/openstack-manuals,saeki-masaki/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,potsmaster/openstack-manuals,openstack/openstack-manuals,saeki-masaki/openstack-manuals,potsmaster/openstack-manuals,openstack/openstack-manuals