Parallels builders: Detect path to Parallels Tools ISO automatically

I've done some job to make Parallels builders more convenient:
* Deprecated `parallels_tools_host_path` template parameter. Now host path to the Parallels Tools ISO is calculating automatically.
* Added `parallels_tools_flavor` template parameter. It is required for calculating the path to Parallels Tools ISO.
* For similarity with other builders the `guest_os_distribution` template parameter has been renamed to `guest_os_type` 
* Updated the documentation
* Added some unit-tests
* Made code refactoring

P.s. I've managed to save backward compatibility as it is possible and also added some informative messages about changed parameters.

cc:/ @rickard-von-essen