Improve security release notes

In order to provide users with more details of 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: I1949a66f894ab3765228d0d210812c875fd59bce