yang-system-test could support recursive search

The original description https://bugs.opendaylight.org/show_bug.cgi?id=6986
mentioned "The system should take folder with yang files",
but most collections of Yang files to test consist of hierarchy if directories.

It would be friendly to user if yang-system-test supported an option to recursively search sub-directories for yang files.
That would also simplify testing for advanced yang-based features (such as binding-parent) which already search recursively.