Rework jobs/playbooks/roles

Current setup doesn't work that well when inheriting. I would like "all
my projects" to only inherit from the job which uses the
`run-test-command` playbook. Then, those jobs which need submodules,
should explicitly ask for that pre- playbook provided by
`git-submodules`.

In Zuul and Ansible, roles are the bits that are supposed to be shared.
As per corvus recommendation on IRC, let's use job variables to control
execution in a more fine-grained manner.

Change-Id: I48c13bff0be7cf796b75e5176774226fb3bb5cc8
diff --git a/roles/run-test-command/meta/main.yaml b/roles/run-test-command/meta/main.yaml
new file mode 100644
index 0000000..7767619
--- /dev/null
+++ b/roles/run-test-command/meta/main.yaml
@@ -0,0 +1,3 @@
+---
+dependencies:
+  - role: zuul-var-to-json
diff --git a/roles/run-test-command/tasks/main.yaml b/roles/run-test-command/tasks/main.yaml
new file mode 100644
index 0000000..1f16aa6
--- /dev/null
+++ b/roles/run-test-command/tasks/main.yaml
@@ -0,0 +1,4 @@
+- name: Run test_command
+  command: '{{ test_command }}'
+  args:
+    chdir: '{{ zuul.project.src_dir }}'