201-vmss-existing-vnet - Key instead of Password

[201-vmss-existing-vnet](https://github.com/Azure/azure-quickstart-templates/tree/master/201-vmss-existing-vnet)

### Issue Details
Deploying with the 201-vmss-existing-vnet template (Linux), it only gives the option of using a Password and not an SSH Key. I don't allow password logins to any of our Linux VM's and I don't see a setting after it is created to switch from Password to Key.

### Repro steps (*if necessary, delete otherwise*)
1. Deploy Template
2. Only have option to set Password

Thanks for any help!! :D

Start in the 201-vmss-existing-vnet template linked in the issue and inspect its deployment parameters and Linux VM configuration. Reproduce the deployment to confirm the available authentication inputs, then verify that the template accepts an SSH key and that a fresh deployment can use key-based login without requiring password authentication.

Written by the indexing model from the issue text.