Support the new path option for yang-model-validator

Support the new path option for yang-model-validator

YANG Tools adopted argparse4j for yang-model-validator in v9.0.0
(YANGTOOLS-1424). The new implementation requires space (' '),
instead of colon (':'), between paths.
More importantly it does not create an aggregated list of YANG
directory paths any more when the path option ('-p', '--path') is used
multiple times - only the last one takes effect.

Update yang-model-validator test suite to support this change in YANG
Tools v9.0.0.

Change-Id: I9307ccf2677702223f221f8f9603ff0ad1ecbff3
Signed-off-by: Sangwook Ha <sangwook.ha@verizon.com>