Fix coverage post_failure stat

Fix ansible syntax error for the coverage job as register
is a higher level action, not a module argument.

Ansible is reporting: Unsupported parameters for (stat) module: register

Change-Id: I69f46b116993c5d55d6b9909c361e07b4e0dc92b

roles/fetch-coverage-output/tasks/main.yaml

@@ -1,7 +1,7 @@
 - name: Check to see if coverage report exists
   stat:
     path: "{{ coverage_output_src }}"
-    register: coverage_report_stat
+  register: coverage_report_stat
 
 - when:
     - zuul_success | default(true) | bool