azure_rm_virtualmachine: enable custom storage account

Attempting to utilize azure_rm_virtualmachine module to define an Azure Virtual Machine with:

- Boot Diagnostics Enabled
- Custom Storage Account in a different Resource Group than the VM
- VM using managed disks

Appears the Ansible module is written in such a way that the VM Boot Diagnostics cannot be enabled with an existing Storage Account that exists outside of the Resource Group of the VM. Noting that this can occur in the Azure Portal/GUI.

> Azure Portal> Target VM > Support + troubleshooting > Boot diagnostics >Settings
> Status: Enable with custom storage account
> Diagnostics storage account drop-down lists possible Storage Accounts that may be utilized

Encounter the following error message from Ansible:

> Error fetching storage account myCustomStorageAccount- Azure Error: ResourceNotFound
>
> Message: The Resource 'Microsoft.Storage/storageAccounts/myCustomStorageAccount' under resource group 'myVMResourceGroup' was not found. For more details please go to https://aka.ms/ARMResourceNotFoundFix

```
- name: Create Azure VM myVMName
azure_rm_virtualmachine:
name: myVMName
resource_group: myVMResourceGroup
managed_disk_type: "Premium_LRS"
boot_diagnostics": {
enabled: true,
storage_account: myCustomStorageAccount
```

```
$ ansible-config dump --only-changed
INTERPRETER_PYTHON(.ansible.cfg) = auto_silent

$ ansible --version
ansible 2.10.0.dev0
config file = .ansible.cfg
configured module search path = [u'.ansible/plugins/modules', u'/usr/share/ansible/plugins/modules']
ansible python module location = /opt/ansible/azure-20200212/lib/python2.7/site-packages/ansible
executable location = /opt/ansible/azure-20200212/bin/ansible
python version = 2.7.5 (default, Aug 13 2020, 02:51:10) [GCC 4.8.5 20150623 (Red Hat 4.8.5-39)]
```