Updated AMI Values that fix bug #24 issue

[sharp99]

Description

I updated the AMI values for palo fw, webserver, ubuntu server with most recent versions. Details of exact versions are also included in the code.

following ami updates made: panos byol 8.1.9.x version dated 08-14-2019 ubuntu xenial image version 16.04 dated 11-14-2019 amazon linux ami amzn-ami-hvm-2018.03.0.20190826-x86_64-gp2 released on 08-26-2019

Motivation and Context

Bug#24

https://github.com/PaloAltoNetworks/terraform-templates/issues/24

How Has This Been Tested?

Entire terraform template in multiple regions has not been tested, yet all images have been verified via the aws cli and are valid/consistent ami id.

Screenshots (if appropriate)

Types of changes

allows code to run as there are now valid ami. current var references non-existent palo alto ami. so currently non-working code.

• Bug fix (non-breaking change which fixes an issue)

Checklist

• [ ] I have updated the documentation accordingly.
• [ x] I have read the CONTRIBUTING document.
• [ x] I have added tests to cover my changes if appropriate.
• [ ] All new and existing tests passed.

[vinayvenkat]

Thank you for the PR. I don't necessarily consider this a bug. I will validate the AMI's and approve the change shortly.

However, intent of terraform is to allow one to customize the templates to suit your specific usecase. Hence the AMI image values are meant to be modified by the users. Thanks again.