do not use unversioned Python binary

We said 'bye' to Python 2 already. So let us use Python 3 for all
scripts.

Unversioned Python binary should be avoided as it may be Python2 or
Python3 (depends on distribution).

Implements: blueprint drop-py2-support

Change-Id: Ifd7912a3debf3ee4ddd8f3e1a4c5870848d4d688

kolla/cmd/build.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.

tools/validate-all-file.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python3
 
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.

tools/validate-yaml.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python3
 
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.

tools/version-check.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.