VM has reported a failure when processing extension 'AzureDiskEncryptionForLinux'. Error message: \"Enable failed.\"."

[101-vm-full-disk-encrypted-rhel](https://github.com/Azure/azure-quickstart-templates/tree/master/101-vm-full-disk-encrypted-rhel)

### Issue Details
I am trying to install the AzureDiskEncryptionForLinux extension on my vm. I have tried on both Ubuntu 16.04 and CentOs 7.2, both of which the docs say are compatible for encryption. While I am able to run all the steps successfully leading up to encryption, the encrypted step consistently fails with
```json

"status": "Failed",
"error": {
"code": "ResourceDeploymentFailure",
"message": "The resource operation completed with terminal provisioning state 'Failed'.",
"details": [
{
"code": "VMExtensionProvisioningError",
"message": "VM has reported a failure when processing extension 'AzureDiskEncryptionForLinux'. Error message: \"Enable failed.\"."
}
]
}
}
```
A few things of note:
1. I am using the Standard_DS2 vm size
2. I am using a key
3. I am testing on a Free trial subscription.

Can someone help/point me in the right direction? Even getting a more sensible error would help immensely.

Start with the linked 101-vm-full-disk-encrypted-rhel template and its AzureDiskEncryptionForLinux deployment step. Reproduce the failure on the stated Ubuntu 16.04 or CentOS 7.2 setup, then determine whether the template or the reported error needs correction; the issue is complete only when the cause and an actionable repository change are established.

Written by the indexing model from the issue text.