Move pycobertura installation into a pre-run

Change-Id: I850b17473f8532e09b6ece4d8f8debb490f7ff59
diff --git a/roles/ensure-pycobertura/tasks/main.yaml b/roles/ensure-pycobertura/tasks/main.yaml
new file mode 100644
index 0000000..473d543
--- /dev/null
+++ b/roles/ensure-pycobertura/tasks/main.yaml
@@ -0,0 +1,4 @@
+- name: Ensure pycobertura is available
+  pip:
+    name: pycobertura
+    extra_args: --user
diff --git a/roles/pycobertura-diff/tasks/main.yaml b/roles/pycobertura-diff/tasks/main.yaml
index f86e739..64726eb 100644
--- a/roles/pycobertura-diff/tasks/main.yaml
+++ b/roles/pycobertura-diff/tasks/main.yaml
@@ -1,8 +1,3 @@
-- name: Ensure pycobertura is available
-  pip:
-    name: pycobertura
-    extra_args: --user
-
 - name: Prepare copy of the new version of sources
   shell: |
     set -ex