Fix rtd publishing

The trigger URL is [0].

The tooling sets rtd_project_name to "airmada" but the project is
named "airship-xxxx" in RTD. Introduce rtd_project_name and set it to
correct value.

See also [1].

[0] https://readthedocs.org/api/v2/webhook/{{ rtd_project_name }}/{{ rtd_webhook_id }}
[1] https://zuul-ci.org/docs/zuul-jobs/general-roles.html#rolevar-trigger-readthedocs.rtd_project_name

Change-Id: I8154ba229addcab2961f9a9c63a8923680ae609b

.zuul.yaml

@@ -15,3 +15,4 @@
       - docs-on-readthedocs
     vars:
       rtd_webhook_id: '47941'
+      rtd_project_name: 'airship-berth'