add yamllint config, fix yamllint errors (#2088)

After a recent change in prometheus/prometheus, Makefile.common includes
now a yamllint target which currently fails. This PR adds the missing
yamllint config and fixes the yamllint errors.

Signed-off-by: Michal Wasilewski <mwasilewski@gmx.com>

.yamllint

@@ -0,0 +1,17 @@
+---
+extends: default
+
+rules:
+  braces:
+    max-spaces-inside: 1
+    level: error
+  brackets:
+    max-spaces-inside: 1
+    level: error
+  commas: disable
+  comments: disable
+  comments-indentation: disable
+  document-start: disable
+  indentation:
+    spaces: consistent
+  line-length: disable