[change] Do not allow applying two (or more) templates of same VPN server on a device

[pandafy]

I found that it is possible to have two different VPN templates for the same VPN server object. But, it is not possible to apply both the templates on the same device, as the JSONSchema generates the following error

There is a conflict with the specified templates. Invalid configuration triggered by "#/files", validator says:

I think we should discourage using different templates for the same VPN server object on the same device using validation (we can create a separate issue for this).

Originally posted by @pandafy in #831 (comment)