Add support for Release Test Automation into build scripts

We need to match offline installers with given RTA keys that define
test sets performed for given offline installer.

Clean up/refactor the offline installer part of the build wrapper.

Change-Id: I52beba890238693cc7c8d3350b04641b326d9775
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>