Removing ACR credentials from variables

[allantargino]

PR #33 introduced Azure Key Vault and Service Connections as a way to remove secrets from group variables in AzDO. This heavily increases security - but the setup also becomes more complicated, given the additional infrastructure requirements that Key Vault introduces. The service connection already all the information we needed to store the service principal credentials, while we were mainly using Key Vault to store ACR credentials.

Main changes:

• This PR removes the dependency of keeping ACR credentials by leveraging data.azurerm_container_registry provider to get credentials at runtime - allowing us to remove Key Vault.
• It has a new step to setup Terraform credentials, extracting all necessary values from the service connection.

Breaking/variables changes:

• Renaming TF_VAR_JMETER_IMAGE_REGISTRY_NAME to TF_VAR_JMETER_ACR_NAME
• Renaming TF_VAR_SERVICE_CONNECTION_NAME to AZURE_SERVICE_CONNECTION_NAME
• Introducing AZURE_SUBSCRIPTION_ID
• Introducing TF_VAR_JMETER_ACR_RESOURCE_GROUP_NAME
• Removing TF_VAR_JMETER_IMAGE_REGISTRY_USERNAME
• Removing TF_VAR_JMETER_IMAGE_REGISTRY_PASSWORD

TF_VAR_* variables are directly used by the terraform template, while AZURE_* are used only by the pipelines.

Other changes:

• Downloading and installing terraform binary during the pipeline. The current version (v.0.13.1) on AzDO managed agents has a bug that affects us (panic: not a collection type (regression between 13.1 and 13.0) hashicorp/terraform#26011)
• Upgrading azurerm provider (fix Upgrade azurerm provider version to 2.26.0 #44)
• Documentation setup improvements. Everything is automated and doesn't require any copy and paste steps.

[allantargino]

cc/ @hepsi204
Please let me know if you have comments or suggestions 👍

[fedeoliv]

@allantargino this approach is great! Getting the credentials at runtime reduces a lot the complexity of managing and configuring Key Vault. LGTM 👍

[hepsi204]

I have to say, this approach is really great and much cleaner than maintaining a key vault. Nice work. 🙂

[allantargino]

Thanks @fernandoBRS!
Also thanks @hepsi204, this is just building on top of your great work :)