openqa_server: try a new approach for checking test changes

basically, dump the config from the server before loading the
templates file, dump it after, and use json_diff to compare.

I wanted to make the actual template loading script from openQA
do some kind of check in '--clean' mode, but given how it's
designed that's not really terribly practical without doing
some major re-architecting, and this seems like a relatively
simple alternative. With this change, the actual "Load tests"
step will never show 'changed', but the subsequent 'Check if
the tests changed in previous step' step will show as changed
when appropriate.

roles/openqa/server/tasks/main.yml

@@ -50,6 +50,7 @@
   - libselinux-python
   - openqa
   - git
+  - json_diff
   - libselinux-utils
   - libsemanage-python
   - nfs-utils
@@ -201,8 +202,23 @@
   when: "gittests|changed"
   changed_when: "1 != 1"
 
-# This will always show as changed, even when nothing in the templates
-# changed; this is a bit hard to fix
+- name: Dump existing config for checking changes
+  shell: "/usr/share/openqa/script/dump_templates --json > /tmp/tmpl-old.json"
+  when: "gittests|changed"
+  changed_when: "1 != 1"
+
+# Because of the boring details of how template loading works, getting
+# a correct 'changed' for this step is too difficult. Instead we have
+# the prior and following steps; when the templates actually changed,
+# the *following* step will register as changed.
 - name: Load tests
   shell: "/tmp/templates --clean"
   when: "gittests|changed"
+  changed_when: "1 != 1"
+
+- name: Check if the tests changed in previous step
+  command: "/usr/share/openqa/script/dump_templates --json > /tmp/tmpl-new.json && json_diff /tmp/tmpl-old.json /tmp/tmpl-new.json"
+  when: "gittests|changed"
+  register: testsdiff
+  changed_when: "testsdiff.rc > 0"
+  failed_when: "1 != 1"