Yamllint fixes (#4410)

* Lint everything in the repository with yamllint

* yamllint fixes: syntax fixes only

* yamllint fixes: move comments to play names

* yamllint fixes: indent comments in .gitlab-ci.yml file

test-infra/image-builder/cluster.yml

@@ -1,4 +1,5 @@
- - hosts: image-builder
-   gather_facts: false
-   roles:
-     - kubevirt-images
+---
+- hosts: image-builder
+  gather_facts: false
+  roles:
+    - kubevirt-images