Add support for unmanaged templates

[andbyrne]

There may be a situation whereby the user would like to create a template directly in vManage, yet be able to reference it from a device template created by this module.

Another example could be when splitting the Terraform state in large environments.

At present, this module requires all referenced resources to be managed by it. There should be a mechanism where data sources are leveraged to provide id and version lookups for resources that are referenced but not managed.