Bug: fix broken spark init command (#486)

Author: jafreck

aztk_cli/config/cluster.yaml

@@ -15,7 +15,7 @@ size: 2
 username: spark
 
 # docker_repo: <name of docker image repo (for more information, see https://github.com/Azure/aztk/blob/master/docs/12-docker-image.md)>
-docker_repo:
+docker_repo: 
 
 # # optional custom scripts to run on the Spark master, Spark worker or all nodes in the cluster
 # custom_scripts: