This commit cleans up some of the work on the Azure ARM provider
following review by @phinze. Specifically:
- Unnecessary ASM-targeted tests are removed
- Validation is added to the `resource_group` resource
- `dns_servers_names` -> `dns_servers` as per the API documentation
- AZURE_SUBSCRIPTION_ID environment variable is renamed to be
ARM_SUBSCRIPTION_ID in order to match the other environment variables
This commit brings some of the work over from #3808, but rearchitects to
use a separate provider for Azure Resource Manager. This is in line with
the decisions made by the Azure Powershell Cmdlets, and is important for
usability since the sets of required fields change between the ASM and
ARM APIs.
Currently `azurerm_resource_group` and `azurerm_virtual_network` are
implemented, more resources will follow.