Move pycobertura installation into a pre-run

Change-Id: I850b17473f8532e09b6ece4d8f8debb490f7ff59
diff --git a/playbooks/coverage-diff/pre.yaml b/playbooks/coverage-diff/pre.yaml
new file mode 100644
index 0000000..e32243a
--- /dev/null
+++ b/playbooks/coverage-diff/pre.yaml
@@ -0,0 +1,3 @@
+- hosts: all
+  roles:
+    - ensure-pycobertura