101-acs-kubernetes - Changing Property `linuxProfile.adminUsername' is not allowed

[101-acs-kubernetes](https://github.com/Azure/azure-quickstart-templates/tree/master/101-acs-kubernetes)

### Issue Details
Copied down the resource manager template and parameters file. Updated the parameters file:

```
{
"$schema": "http://schema.management.azure.com/schemas/2015-01-01/deploymentParameters.json#",
"contentVersion": "1.0.0.0",
"parameters": {
"dnsNamePrefix": {
"value": "REDACTED-acs-test"
},
"adminUsername": {
"value": "test"
},
"sshRSAPublicKey": {
"value": "REDACTED"
},
"servicePrincipalClientId": {
"value": "REDACTED"
},
"servicePrincipalClientSecret": {
"value": "REDACTED"
},
"orchestratorType":{
"value": "Kubernetes"
}
}
}
```
Ran the command

```PS> az group deployment create --resource-group testAcsTempl
ateGroup --template-file .\kubernetes-resource-manager-template.json --parameters .\kubernetes-resource-manager-template-vars.json
```

Error message received:

```
Deployment failed. {
"error": {
"code": "PropertyChangeNotAllowed",
"target": "linuxProfile.adminUsername",
"message": "Changing property 'linuxProfile.adminUsername' is not allowed."
}
}
```

Tried changing the adminUserName to 'admin', received message:

```
Deployment failed. {
"status": "Failed",
"error": {
"code": "ResourceDeploymentFailure",
"message": "The resource operation completed with terminal provisioning state 'Failed'.",
"details": [
{
"code": "ProvisioningFailed",
"message": "Provisioning of resource(s) for container service 'containerservice-testAcsTemplateGroup' in resourc
e group 'testAcsTemplateGroup' failed with errors: Resource type: Microsoft.Compute/virtualMachines, name: k8s-master-DD
F7E832-0, id: /subscriptions/2a04bef4-ed1a-4d99-8823-f9cca41b3aba/resourceGroups/testAcsTemplateGroup/providers/Microsof
t.Compute/virtualMachines/k8s-master-DDF7E832-0, StatusCode: BadRequest, StatusMessage: \\n {\r\n \"error\": {\r\n \
"code\": \"InvalidParameter\",\r\n \"target\": \"osDisk\",\r\n \"message\": \"Cannot specify user image overrides
for a disk already defined in the specified image reference.\"\r\n }\r\n}\r\nResource type: Microsoft.Compute/virtualMa
chines, name: k8s-agent-DDF7E832-0, id: /subscriptions/2a04bef4-ed1a-4d99-8823-f9cca41b3aba/resourceGroups/testAcsTempla
teGroup/providers/Microsoft.Compute/virtualMachines/k8s-agent-DDF7E832-0, StatusCode: BadRequest, StatusMessage: \\n {\r
\n \"error\": {\r\n \"code\": \"InvalidParameter\",\r\n \"target\": \"adminUsername\",\r\n \"message\": \"The
Admin Username specified is not allowed.\"\r\n }\r\n}\r\n"
}
]
}
}
```

Start by reviewing 101-acs-kubernetes/kubernetes-resource-manager-template.json and its parameters file, then reproduce the deployment with az group deployment create. Inspect the linuxProfile.adminUsername, VM image reference, and osDisk details named in the errors; done means the template deploys without the reported property, image, or username failures.

Written by the indexing model from the issue text.