gsyncd: allow the override of the compiled-in python path

.. using the environment variable $PYTHON

Change-Id: Ieaad8be98b826c803268216826e250d9944c8190
BUG: 882127
Signed-off-by: Joe Julian <me@joejulian.name>
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/4252
Tested-by: Gluster Build System <jenkins@build.gluster.com>