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

tests/files/gce_ubuntu-cilium-sep.yml

@@ -1,3 +1,4 @@
+---
 # Instance settings
 cloud_image_family: ubuntu-1604-lts
 cloud_region: us-central1-b
@@ -9,4 +10,3 @@ deploy_netchecker: true
 enable_network_policy: true
 dns_min_replicas: 1
 cloud_provider: gce
-