LU-15415 tests: Use correct PYTHON in Makefile.am

LU-15415 tests: Use correct PYTHON in Makefile.am

Use already defined PYTHON from autoMake, instead of trying to
guess correct one using undefined PYTHON_VERSION.

Test-Parameters: trivial
Signed-off-by: Nathaniel Clark <nclark@whamcloud.com>
Change-Id: I44e2e5d44121c0bd39cf3032ae228bd6cd442a8c
Reviewed-on: https://review.whamcloud.com/45989
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Minh Diep <mdiep@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Chris Horn <chris.horn@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>