Use python instead of python2

On used docker baseimage python is already linked to python2 or python3,
depending on image version

backend-sample-app.py

 #!/bin/sh
-''''which python2 >/dev/null && exec python2 "$0" "$@" # '''
 ''''which python  >/dev/null && exec python  "$0" "$@" # '''
 
 # Copyright (C) 2014-2015 Nginx, Inc.

nginx-ldap-auth-daemon.py

 #!/bin/sh
 ''''[ -z $LOG ] && export LOG=/dev/stdout # '''
-''''which python2 >/dev/null && exec python2 -u "$0" "$@" >> $LOG 2>&1 # '''
 ''''which python  >/dev/null && exec python  -u "$0" "$@" >> $LOG 2>&1 # '''
 
 # Copyright (C) 2014-2015 Nginx, Inc.