[SP-1592] - Backport of BISERVER-12205 - User security gets corrupted

Main discussion thread - http://jira.pentaho.com/browse/ESR-4197
Implemented suggested approach: reverted changes in MondrianAbstractPlatformUserRoleMapper class. Now roles will be pulled from the session. 
Unit tests will be soon.
@pentaho-nbaker Nick, please review.