Prefer --fast for skipping all checks

Closes https://git.autonomic.zone/coop-cloud/abra/issues/169

.drone.yml

@@ -32,6 +32,10 @@ steps:
       - run shellcheck
       - run flake8
       - run unit tests
+    when:
+      event:
+        exclude:
+          - pull_request
 
   - name: trigger downstream builds
     image: plugins/downstream
@@ -47,6 +51,10 @@ steps:
       - run flake8
       - run unit tests
       - publish image
+    when:
+      event:
+        exclude:
+          - pull_request
 
   - name: notify on failure
     image: plugins/matrix