Integration tests related improvements

* Field "help" for parameter FLAVOR_ID in file config.py was updated;
* Comments to some parameters in file itest.conf.sample-full were added;
* Exceptions for image related methods were updated and added new;
* Symbols "*" was cut from exceptions.

Change-Id: I1924ba7979ee27976ffd71e1087c51b9ca5311d6
Merge "Integration tests related improvements"