refactor(example/gwlb_with_vmseries): Refactor Gateway Load Balancer example

[sebastianczech]

Description

PR is a part of #9 and it delivers Gateway Load Balancer in refactored example gwlb_with_vmseries.

Motivation and Context

In #3 all modules and examples are being refactored according to issue description. PR 372 in archived repository delivered changes in gwlb module, but it didn't contain refactored example with Gateway Load Balancer.

How Has This Been Tested?

Code was tested by deploying example in the lab.

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist

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

[alperenkose]

IMO we should keep Common and GWLB examples separate as GWLB is not in the reference architecture design guide yet. We also have links from pan.dev to the repository examples for these designs so merging them with different example.tfvars can cause confusion.

General question; if we keep the GWLB example separate shall we still keep the un-compatible modules in the example code like NATGW or AppGW ? I believe these resources are not possible to integrate with GWLB, and it might bring confusion/doubts to the users trying out the example.

[acelebanski]

So summing up today's discussion during the planning meeting, we decided not to share common codebase with GWLB example, it will remain separate.

We will also transform the test_infrastructure example into a module and then integrate it with all examples but for now (in scope of this PR), I think the test infra can remain a part of the GWLB example.

[sebastianczech]

thanks @alperenkose for review, I updated the code, there is no Application Gateway and NAT Gateway in the main.tf.

@acelebanski I removed changes from common design and I left refactored example for GWLB together test VMs.

PR is ready to being reviewed.