provider/azurerm: avoid registering providers already registered, add option to disable
provider/azurerm: avoid registering providers already registered, add option to disable

The provider now consults the Providers API to detect which providers are already
registered and uses this call to test the credentials

A new option `skip_provider_registration` / `ARM_SKIP_PROVIDER_REGISTRATION` is
now available to opt out of provider registration entirely

sebastus/terraform-provider-azurerm,sebastus/terraform-provider-azurerm,thiagocaiubi/terraform-provider-azurerm,thiagocaiubi/terraform-provider-azurerm,abhijeetgaiha/terraform-provider-azurerm,abhijeetgaiha/terraform-provider-azurerm,thiagocaiubi/terraform-provider-azurerm,abhijeetgaiha/terraform-provider-azurerm,sebastus/terraform-provider-azurerm