yamllint: Make all files in awx pass yamllint

This commit updates all files that weren't passing yamllint for them to
pass.

A new yamllint target has been added. One can run `tox -e yamllint` or
`yamllint -s .` locally to ensure yaml files are still passing.

This check will be enabled in the CI so it can get on every new
contributions, and prevent merging non-compliant code.

Signed-off-by: Yanis Guenane <yguenane@redhat.com>

tools/docker-credential-plugins-override.yml

@@ -1,3 +1,4 @@
+---
 version: '2'
 services:
   # Primary Tower Development Container link
@@ -21,7 +22,8 @@ services:
     environment:
       DATABASE_URL: postgres://postgres@postgres/postgres
       CONJUR_DATA_KEY: 'dveUwOI/71x9BPJkIgvQRRBF3SdASc+HP4CUGL7TKvM='
-    depends_on: [ postgres ]
+    depends_on:
+      - postgres
     links:
       - postgres
     ports: