First take on restructuring the terraform directory:

• Remove old terraform configurations that are no longer needed. Move the azarm config to terraform/azarm - azarm is still used in ghaf github action build pipeline as aarch64 remote builder machine.
• terraform/main.tf now describes azure infra configuration based on Florian's PR#35
• The tree VM configurations originally described in Florian's PR are now directly under terraform directory, at terraform/jenkins-controller.tf, terraform/binary-cache.tf, and terraform/builder.tf
• Modules used by those three VM configs are moved under terraform/modules
• terraform/state-storage describes the shared tfstate storage used to store the terraform state on Azure
• terraform/persistent describes the persistent data, not removed when development environments (ghaf-infra instances) are removed. Currently, such persistent data includes binary cache signing key, builder ssh key, and binary cache storage. 'dev' environments share one instance of such persistent data, whereas, prod environment(s) will have their own set of keys and binary cache storage.
• This PR also includes a script init-ghaf-infra.sh to automate the initial setup of the state-storage and persistent data

Relevant documentation needs to be re-written, which will be done with a follow-up PR.