ipsec: Use @PYTHON@ directly instead of "/usr/bin/env python"

Using "/usr/bin/env" is against Fedora Packaging Guidelines [1].

Moreover, in this specific case, it also prevent "make rpm-fedora" to
successfully complete on "Fedora Rawhide" since "#!/usr/bin/env python"
must not be used anymore [2].

[1] https://fedoraproject.org/wiki/Packaging:Guidelines#Shebang_lines
[2] https://fedoraproject.org/wiki/Changes/Make_ambiguous_python_shebangs_error

CC: Qiuyu Xiao <qiuyu.xiao.qyx@gmail.com>
Fixes: 22c5eafb6e ("ipsec: reintroduce IPsec support for tunneling")
Signed-off-by: Timothy Redaelli <tredaelli@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>

configure.ac

@@ -208,6 +208,7 @@ AC_CONFIG_COMMANDS([include/openflow/openflow.h.stamp])
 AC_CONFIG_COMMANDS([utilities/bugtool/dummy], [:])
 AC_CONFIG_COMMANDS([ovn/dummy], [:])
 AC_CONFIG_COMMANDS([ovn/utilities/dummy], [:])
+AC_CONFIG_COMMANDS([ipsec/dummy], [:])
 
 m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES])
 

ipsec/automake.mk

@@ -5,6 +5,6 @@
 # notice and this notice are preserved.  This file is offered as-is,
 # without warranty of any kind.
 
-EXTRA_DIST += \
-        ipsec/ovs-monitor-ipsec
-FLAKE8_PYFILES += ipsec/ovs-monitor-ipsec
+bin_SCRIPTS += ipsec/ovs-monitor-ipsec
+EXTRA_DIST += ipsec/ovs-monitor-ipsec.in
+FLAKE8_PYFILES += ipsec/ovs-monitor-ipsec.in

ipsec/ovs-monitor-ipsec.in

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#! @PYTHON@
 # Copyright (c) 2017 Nicira, Inc.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");