Remove $NTA reference

Supports https://github.com/numenta/nupic/issues/809.

This PR should be merged together with https://github.com/numenta/nupic/pull/917 (pip-installable). This is because `lib_cppregion.so` will be installed too. In the current implementation, this file is obtained from $NTA/lib.
> I guess the code is not working like this, stderr is likely to be omitted by OS::executeCommand(), so you can't access to the error output?

What is returned is `stdout`, not `stderr`. In this case, `stdout` is the path returned by the shell command. Python scripts only will return some path if NuPIC really is installed, if not installed, it will return empty. If empy, `if (!Path::exists(rootDir_))` condition will return error.