Improve security section of release notes

Improve security section of release notes

In order to provide users with more details on project's state in
terms of security let's divide the security release notes into three
sections:

- Fixed Security Issues
  Contains a list of security fixes merged during this
  release (especially those reported via OJSI tickets).

- Known Security Issues
  Contains a list of vulnerabilities detected in project during
  release which have not been fixed yet and thus should be mitigated
  by the user.

- Known Vulnerabilities in Used Modules
  Contains information about NexusIQ scan results

Issue-ID: SECCOM-238
Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>
Change-Id: I8bbac2b7e7126369e30da218b69cdc3744d3c0c5