* Add support for `terraform-provider-azurerm` v2.0+. Require
`terraform-provider-azurerm` v2.0+ and drop v1.x support since
the Azure provider major release is not backwards compatible
* Use Azure's new Linux VM and Linux VM Scale Set resources
* Change controller's Azure disk caching to None
* Associate subnets (in addition to NICs) with security groups
(aesthetic)
* If set, change `worker_priority` from `Low` to `Spot` (action required)
Related:
* https://www.terraform.io/docs/providers/azurerm/guides/2.0-upgrade-guide.html
* Update terraform-render-bootstrap module to adopt the
Terrform v0.12 templatefile function feature to replace
the use of terraform-provider-template's `template_dir`
* Require Terraform v0.12.6+ which adds `for_each`
Background:
* `template_dir` was added to `terraform-provider-template`
to add support for template directory rendering in CoreOS
Tectonic Kubernetes distribution (~2017)
* Terraform v0.12 introduced a native `templatefile` function
and v0.12.6 introduced native `for_each` support (July 2019)
that makes it possible to replace `template_dir` usage
* Allow updating terraform-provider-ct to any release
beyond v0.3.2, but below v1.0. This relaxes the prior
constraint that allowed only v0.3.y provider versions
* Replace v0.11 bracket type hints with Terraform v0.12 list expressions
* Use expression syntax instead of interpolated strings, where suggested
* Update Azure tutorial and worker pools documentation
* Define Terraform and plugin version requirements in versions.tf
* Require azurerm ~> 1.27 to support Terraform v0.12
* Require ct ~> 0.3.2 to support Terraform v0.12
