Add Capabilities to deploy GKE Jmeter on existing VPC

[gcpjaru]

We would like to deploy Jmeter GKE on existing VPC for security and networking design practices.

[manitgupta]

Machmeter creates a VPC and a subnet inside it using Terraform. Terraform does not have the capability to detect existing resources which do not exist in the .tfstate file.
In order to support this, a different Terraform template will be required which does not attempt to create a VPC network and subnet and instead uses the user specified value.

Currently Machmeter does not support multiple Terraform templates basis the user configuration, and instead deploys a single template.

We will need to look into constructing terraform templates based on the user input to support this requirement. This will likely require restructuring portions of the code base.