Code refactoring and improvements for MuranoPL testing mini-framework

* Stack-traces of caught exceptions are no longer printed in Python 2.6
* No more warning on Exception.message being deprecated
* Loaded YAML files are cached instead of being re-read for each test
   greatly increasing performance without side effects because class-loader
   is still new on each test
* object_model helper objects can be used as a test method arguments
* Path to core library classes was relative to the test. That prevented grouping
   tests into sub-folders
* Preserve original stack-trace for Python exceptions thrown by the engine
* Ability to delete (clear) traces (produced by calls to trace(...) in MuranoPL code)
* Code refactoring

Change-Id: Ifab8460d3a188c6e05edd682a302e2a8a6dd0425
Merge "Code refactoring and improvements for MuranoPL testing mini-framework"