Make Python interpreter selectable for test scripts

The Python interpreter used to run the test cases is hard-coded to be
/usr/bin/python. If we use the first one from $PATH instead, it is
much easier to test ganeti with other Python versions.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>

Makefile.am

@@ -222,7 +222,7 @@ nodist_TESTS =
 
 TESTS = $(dist_TESTS) $(nodist_TESTS)
 
-TESTS_ENVIRONMENT = PYTHONPATH=.:$(top_builddir)
+TESTS_ENVIRONMENT = PYTHONPATH=.:$(top_builddir) $(PYTHON)
 
 RAPI_RESOURCES = $(wildcard lib/rapi/*.py)