name ec2 instances and security groups consistently

Security groups created by spark-ec2 do not prepend “spark-“ to the
name.

Since naming the instances themselves is new to spark-ec2, it’s better
to change that pattern to match the existing naming pattern for the
security groups, rather than the other way around.

ec2/spark_ec2.py

@@ -428,11 +428,11 @@ def launch_cluster(conn, opts, cluster_name):
     for master in master_nodes:
         master.add_tag(
             key='Name',
-            value='spark-{cn}-master-{iid}'.format(cn=cluster_name, iid=master.id))
+            value='{cn}-master-{iid}'.format(cn=cluster_name, iid=master.id))
     for slave in slave_nodes:
         slave.add_tag(
             key='Name',
-            value='spark-{cn}-slave-{iid}'.format(cn=cluster_name, iid=slave.id))
+            value='{cn}-slave-{iid}'.format(cn=cluster_name, iid=slave.id))
 
     # Return all the instances
     return (master_nodes, slave_nodes)