Add yamllint configuration, run yamllint in Travis

This ensures all yaml files we have in cluster-monitoring-operator
are parsable and consistently indented.

.travis.yml

@@ -5,8 +5,12 @@ go:
 services:
 - docker
 before_install:
+- sudo apt-get -qq update
+- sudo apt-get install -y python-pip
+- sudo pip install yamllint
 jobs:
   include:
+  - script: yamllint .
   - stage: Build
     script: make build
   - script: make generate && git diff --exit-code

.yamllint

@@ -0,0 +1,15 @@
+---
+extends: default
+ignore: | 
+ vendor/
+ manifests/cluster-monitoring-operator-role.yaml
+
+rules: 
+  indentation:
+    spaces: 2
+    indent-sequences: consistent
+  document-start: disable
+  line-length: disable
+  trailing-spaces: disable
+  colons: disable
+  empty-lines: disable