[RT-872] Use the custom service account name if present

[Stuart133]

We allow a user to specify their own service account rather than having one created by the chart but due to a template bug this name was not getting passed to the deployment which means the default service account is mounted.

This PR means the service account will always be templated, with a default value of default if create is turned off & no service account name is set

Testing

The change was manually tested with helm template to verify the expected output (Shown below)

Before

The relevant snippet from the rendered helm template

  spec:
      terminationGracePeriodSeconds: 18300
      volumes:
        - name: taskpod-config
          configMap:
            name: yeah-container-agent

After

  spec:
      serviceAccountName: custom-name
      terminationGracePeriodSeconds: 18300
      volumes:
        - name: taskpod-config
          configMap:
            name: yeah-container-agent