Additional validation of python and perl custom tools when "launcherId" is not specified

Prerequisites:

1. Make sure python2 is installed and the path to it is in the PATH environment variable.

Scenario:

1. Download the attached files, put them into the same folder.
2. Open the "External tools" page in the "Application Settings" dialog.
3. In the "Custom tools" group click "Import", select the downloaded *.xml config.
   Current result: there is a warning sign on the icon for the tool. The tool didn't pass validation.
   Expected result: the tool is valid (note that the system python is used for the validation).
4. Open the WD.
5. Create workflow custom commandline element with added tool.
6. Run workflow.
   Expected result: workflow finished without errors

Later when the tool is used in an element in a workflow, system python should also be used.

The same should work for *.pl files and perl interpreter, available in PATH.