[PDI-12413] Reverting SpringSecurityPrincipalProvider enhancements due to bad Authentication objects being re-used (still unknown why). Also PentahoSystem.runAsSystem now behaves like SecurityHelper.runAsSystem
Merge pull request #1631 from pentaho-nbaker/5.1

[PDI-12413] Reverting SpringSecurityPrincipalProvider enhancements due t...