Convert nova-next to a zuul v3 job

For the most part this should be a pretty straight-forward
port of the run.yaml. The most complicated thing is executing
the post_test_hook.sh script. For that, a new post-run playbook
and role are added.

The relative path to devstack scripts in post_test_hook.sh itself
had to drop the 'new' directory since we are no longer executing
the script through devstack-gate anymore the 'new' path does not
exist.

Change-Id: Ie3dc90862c895a8bd9bff4511a16254945f45478

roles/run-post-test-hook/README.rst

@@ -0,0 +1,8 @@
+Runs gate/post_test_hook.sh.
+
+**Role Variables**
+
+.. zuul:rolevar:: devstack_base_dir
+   :default: /opt/stack
+
+   The devstack base directory.

roles/run-post-test-hook/defaults/main.yaml

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+devstack_base_dir: /opt/stack

roles/run-post-test-hook/tasks/main.yaml

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+- name: Run post_test_hook.sh
+  command: "{{ devstack_base_dir }}/nova/gate/post_test_hook.sh"
+  become: yes