Naming convention of hostnames and azure VM objects

_From @fberson on December 1, 2016 14:49_

The naming os computer hostnames and azure VM names is not very consistent and does not allow customers to add a custom prefix. I would like to suggest introduce 3 new variables to be able to provide the prefix for each of the three roles, as an example for RD Gateway:
```
"gatewayHostNamePrefix": {
"type": "string",
"defaultValue": "RDGW-",
"metadata": {
"description": "Hostname prefix for RD Gateway Servers"
}
```
Use those to construct the computername like below
```
"osProfile": {
"computerName": "[concat(parameters(' gatewayHostNamePrefix'),'0', copyindex(1))]",

```
And use the same convention for the VM names in Azure as well to make things consistent
```
"type": "Microsoft.Compute/virtualMachines",
"name": "[concat(parameters(gatewayHostNamePrefix'),'0', copyindex(1))]",
```

_Copied from original issue: mmarch/rds-templates#13_