Updates to documentation for Release 2.6.0

Updates to Admin manual et al. Fixes for:
https://issues.cask.co/browse/CDAP-603
https://issues.cask.co/browse/CDAP-612
https://issues.cask.co/browse/CDAP-613
https://issues.cask.co/browse/CDAP-650
https://issues.cask.co/browse/CDAP-655

Staged at: http://stg-web101.sw.joyent.continuuity.net/cdap/2.6.0-SNAPSHOT-2.6.0_doc_updates/en/admin-manual/installation/security.html#testing-security