fix(deis-slugbuilder.yaml): use repo_name instead of deis-slugbuilder

so the builder script can replace it with the git sha, and no
collisions occur

Author: arschles

rootfs/etc/deis-slugbuilder.yaml

@@ -1,7 +1,7 @@
 apiVersion: v1
 kind: Pod
 metadata:
-  name: deis-slugbuilder
+  name: repo_name
   labels:
     heritage: deis
     version: v2-alpha