allow subnets without nsg on Firewall, Bastion and Gateway

[withstu]

Exclude subnets that do not allow NSGs to be attatched to. Including Firewall, Bastion and Gateway.

solves #38

[ghost]

All CLA requirements met.

[jtracey93]

Thanks @withstu I have actually updated this in the ESLZ repo where we take the policies from for the Terraform module.

@krowlandson when are we next planning a policy pull?

[krowlandson]

@jtracey93 we can run a policy update and release a new patch version at any point for updates like this. Let's catch up tomorrow to see how quick we can push this out. Assume your update in ES covers all of the above?

[krowlandson]

Also, just to say thank you @withstu for your contribution. Even if we don't merge this PR, we will use this to trigger an update from upstream which is due anyway :)

[krowlandson]

@withstu, I believe this should now be covered by the new excludedSubnets parameter added to this policy in the latest release v0.4.0 so I will close this PR.

Please check out the upgrade notes as this update includes some significant changes.

If you have any further questions, please reach out and we'll be happy to help.