perf tools: Default to python version 2

perf tools: Default to python version 2

According to PEP 394 recommendation [1], it's more portable to use
python2 rather than plain python to refer python binary version 2.

Since there're distros using python3 by default like Arch, and we don't
support python3 (yet), it'd be better using python2 explicitly.

But older versions (prior to 2.7) seem not to provide python2 but just
python.  Given that it's only old version, try python2 first and then
fallback to python.  It'll ensure that it always points to python 2.x.

I tested (compiles and perf script runs) with the combinations:

1) python  -> python2.x, python-config  -> python2.x-config
   python2 N/A,          python2-config N/A

2) python  -> python3.x, python-config  -> python3.x-config
   python2 -> python2.x, python2-config -> python2.x-config

3) python  -> python2.x, python-config  -> python2.x-config
   python2 -> python2.x, python2-config -> python2.x-config

4) python  -> python2.x, python-config  -> python2.x-config
   python2 -> python2.x, python2-config N/A

Based on / replaces the patch 2/2 by Namhyung Kim.

[1] https://www.python.org/dev/peps/pep-0394

Based-on-patch-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Thomas Ilsche <thomas.ilsche@tu-dresden.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/53DF8493.6070206@tu-dresden.de
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>