Changing property 'linuxConfiguration.ssh.publicKeys' is not allowed

guacamole-rdp-vnc-gateway-existing-vnet [https://github.com/Azure/azure-quickstart-templates/tree/master/guacamole-rdp-vnc-gateway-existing-vnet](url)

### Issue Details
Various errors in deployment. Worked through the username problem, however I can not get past the SSH key
Tried the one created in PuttyGen (publicKey for pasting into Open SSH authorized_keys file) and also the contents of the publickey saved via PuttyGen (file starts with ---- BEGIN SSH2 PUBLIC KEY ----
Comment: "rsa-key-guacamole". then the key data)

Activity Log Summary is:
Operation name
Create or Update Virtual Machine
Time stamp
Fri Dec 14 2018 16:35:33 GMT+0000 (Greenwich Mean Time)
Event initiated by
me @ myAzureEmailAddress
Error code
PropertyChangeNotAllowed
Message
Changing property 'linuxConfiguration.ssh.publicKeys' is not allowed.

Start with the guacamole-rdp-vnc-gateway-existing-vnet template linked in the issue and reproduce the deployment using the Activity Log details. Compare the SSH public key inputs described in the report and determine the supported format and change needed; done means the deployment no longer reports PropertyChangeNotAllowed.

Written by the indexing model from the issue text.