Install Python 3 version of aws-cfn-bootstrap scripts on CentOS 7 and Ubuntu 18.04

The latest release (version 0.6.0) of [pystache](https://pypi.org/project/pystache/) removed the support for Python2.
This package is a dependency of aws-cfn-bootstrap-latest.tar.gz.

The latest (version 1.4-34) [aws-cfn-bootstrap-latest.tar.gz](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/releasehistory-aws-cfn-bootstrap.html#releasehistory-aws-cfn-bootstrap-v1) became incompatible with Python2 after the pystache update, and it is recommended in [CloudFormation helper scripts reference](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/cfn-helper-scripts-reference.html) to use the latest installation package for Python3.

Note: this package is not needed on Amazon Linux 2 because CFN helpers scripts are already included in the AMIs
and we are already using Python3 version for Ubuntu 20.04.

Signed-off-by: Hanwen <hanwenli@amazon.com>
Signed-off-by: Enrico Usai <usai@amazon.com>

Author: hanwen-cluster

CHANGELOG.md

@@ -15,6 +15,7 @@ This file is used to list changes made in each version of the AWS ParallelCluste
 - Upgrade CUDA library to version 11.4.3.
 - Upgrade NVIDIA Fabric manager to `nvidia-fabricmanager-470`.
 - Disable unattended upgrades for Ubuntu.
+- Install Python 3 version of `aws-cfn-bootstrap` scripts on CentOS 7 and Ubuntu 18.04.
 
 2.11.3
 -----

amis/packer_centos7.json

@@ -298,9 +298,8 @@
         "region=\"{{user `region`}}\"",
         "bucket=\"s3.amazonaws.com\"",
         "[[ ${region} =~ ^cn- ]] && bucket=\"s3.cn-north-1.amazonaws.com.cn/cn-north-1-aws-parallelcluster\"",
-        "curl --retry 3 -L -o /tmp/aws-cfn-bootstrap-latest.tar.gz https://${bucket}/cloudformation-examples/aws-cfn-bootstrap-latest.tar.gz",
-        "which pip2",
-        "if [ $? -eq 0 ]; then sudo pip2 install /tmp/aws-cfn-bootstrap-latest.tar.gz; else sudo pip install /tmp/aws-cfn-bootstrap-latest.tar.gz; fi"
+        "curl --retry 3 -L -o /tmp/aws-cfn-bootstrap-py3-latest.tar.gz https://${bucket}/cloudformation-examples/aws-cfn-bootstrap-py3-latest.tar.gz",
+        "sudo pip3 install /tmp/aws-cfn-bootstrap-py3-latest.tar.gz"
       ]
     },
     {

amis/packer_ubuntu1804.json

@@ -295,8 +295,8 @@
         "region=\"{{user `region`}}\"",
         "bucket=\"s3.amazonaws.com\"",
         "[[ ${region} =~ ^cn- ]] && bucket=\"s3.cn-north-1.amazonaws.com.cn/cn-north-1-aws-parallelcluster\"",
-        "curl --retry 3 -L -o /tmp/aws-cfn-bootstrap-latest.tar.gz https://${bucket}/cloudformation-examples/aws-cfn-bootstrap-latest.tar.gz",
-        "sudo pip install /tmp/aws-cfn-bootstrap-latest.tar.gz"
+        "curl --retry 3 -L -o /tmp/aws-cfn-bootstrap-py3-latest.tar.gz https://${bucket}/cloudformation-examples/aws-cfn-bootstrap-py3-latest.tar.gz",
+        "sudo pip3 install /tmp/aws-cfn-bootstrap-py3-latest.tar.gz"
       ]
     },
     {