Makefile.am: fix permissions for Python scripts on install

Some Python scripts in /usr/lib/ganeti/ were getting the wrong permissions
(their 'x' bit was cleared).  This patch fixes that behavior.

This patch renames the variable 'dist_tools_PYTHON' to 'python_scripts'.
Some Python scripts were listed in the 'dist_tools_PYTHON' variable, but as
said scripts have no .py extension in their names, Automake treated the scripts
as data files, and hence no 'x' bit.  Now the Python scripts are processed
by the rules created for the 'dist_tools_SCRIPTS' variable, and such rules
don't depend on file name extensions.

Signed-off-by: Bernardo Dal Seno <bdalseno@google.com>
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>