Documentation for arm_management_group resource

Remove required, computed on subscription_ids
Made CreateManagementGroupRequest properties lean
Remove unnecessary evaluation of future result
Modified test case with subscription to manipulate subscriptions
Fixed up documentation page label
Removed unnecessary examples

- Allowing users to specify the Group ID
- Renaming `name` to `display_name` / making it optional
- Generating a Group ID / Display Name if not specified
- Support for Parent Management Groups
- Additional tests

Tests pass:

```
 $ acctests azurerm TestAccAzureRMManagementGroup_

=== RUN   TestAccAzureRMManagementGroup_basic
--- PASS: TestAccAzureRMManagementGroup_basic (56.52s)
=== RUN   TestAccAzureRMManagementGroup_nested
--- PASS: TestAccAzureRMManagementGroup_nested (86.19s)
=== RUN   TestAccAzureRMManagementGroup_multiLevel
--- PASS: TestAccAzureRMManagementGroup_multiLevel (122.92s)
=== RUN   TestAccAzureRMManagementGroup_multiLevelUpdated
--- PASS: TestAccAzureRMManagementGroup_multiLevelUpdated (200.95s)
=== RUN   TestAccAzureRMManagementGroup_withName
--- PASS: TestAccAzureRMManagementGroup_withName (68.17s)
=== RUN   TestAccAzureRMManagementGroup_updateName
--- PASS: TestAccAzureRMManagementGroup_updateName (98.42s)
=== RUN   TestAccAzureRMManagementGroup_withSubscriptions
--- PASS: TestAccAzureRMManagementGroup_withSubscriptions (189.71s)
PASS
ok  	github.com/terraform-providers/terraform-provider-azurerm/azurerm	823.213s
```