Add a security.openstack.org site for advisories

This is the Apache vhost definition providing a place to host
security advisory documents, which will be auto-published in a
continuous fashion from files in the openstack/ossa repo.

Change-Id: I13e4e9731345d629854b4c11efeac8fe2f6b6058
Merge "Add a security.openstack.org site for advisories"