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.
Nashwan Azhari