pin etcd-mixin to last working version for release

etcd refactored their repo moving and renaming etcd-mixin. the
jsonnetfile depended on "master" even though the lock was for an older
version. checking out from the last commit before the move works.

jsonnet/kube-prometheus/jsonnetfile.json

@@ -17,7 +17,7 @@
           "subdir": "Documentation/etcd-mixin"
         }
       },
-      "version": "master"
+      "version": "e8ba375032e8e48d009759dfb285f7812e7bcb8c"
     },
     {
       "source": {