Description of problem:
The issues we encountered were caused by incorrect documentation.
We suggest that you forward the problem we faced to the appropriate people responsible for the documentation

We tried  the exact example described in section 5.4 of this document:
https://access.redhat.com/documentation/en/red-hat-openstack-platform/8/partner-integration/partner-integration

There are two problems with that example
1) The problem you solved for us:
    the type of the server parameter should be "string" and not "json"
2) The identation of the properties field in the ExtraPreDeployment resource is incorrect
   This problem, we were able to solve ourselves

The correct documentation for this is in:

As outlined in our documentation:
https://access.redhat.com/documentation/en/red-hat-openstack-platform/8/single/director-installation-and-usage/

The file needs to look like this:
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[stack@undercloud-3 ~]$ cat templates/nameserver.yaml 
heat_template_version: 2014-10-16

parameters:
  server:
    type: string
  nameserver_ip:
    type: string

resources:
  ExtraPreConfig:
    type: OS::Heat::SoftwareConfig
    properties:
      group: script
      config:
        str_replace:
          template: |
            #!/bin/sh
            echo "nameserver _NAMESERVER_IP_" >> /etc/resolve.conf
          params:
            _NAMESERVER_IP_: {get_param: nameserver_ip}
  ExtraPreDeployment:
    type: OS::Heat::SoftwareDeployment
    properties:
      config: {get_resource: ExtraPreConfig}
      server: {get_param: server}
      actions: ['CREATE']

outputs:
  deploy_stdout:
    description: Deployment reference, used to trigger post-deploy on changes
    value: {get_attr: [ExtraPreDeployment, deploy_stdout]}


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