Add OSSN-0011 - Heat templates with invalid references allows unintended network access

This adds OSSN-0011, which covers an issue related to invalid
security group references in CFN templates being improperly evaluated
by Heat.  This results in unintended network access being allowed.

Related-Bug: 1291091

Change-Id: I88ee23aadc74020f150332a619796ebd77ef9698