packer build fails when a template name is provided for the source_template option

Documentation for the source_template option in the cloudstack source block at https://developer.hashicorp.com/packer/plugins/builders/cloudstack says that "the **name** or ID of the template used as base template for the instance" is allowed. When using the name of the template, packer returns this error:

Error retrieving UUID of template : No match found for : &{Count:0 Templates:[]}

Only the ID of a template appears to be accepted. Need to be able to specify template name as well.

Start with the CloudStack source block's source_template handling and reproduce the failure using a template name instead of an ID. The change is done when a template name is accepted as documented, while ID-based configuration continues to work.

Written by the indexing model from the issue text.