[SPARK-2065] give launched instances names

This update resolves [SPARK-2065](https://issues.apache.org/jira/browse/SPARK-2065). It gives launched EC2 instances descriptive names by using instance tags. Launched instances now show up in the EC2 console with these names.

I used `format()` with named parameters, which I believe is the recommended practice for string formatting in Python, but which doesn’t seem to be used elsewhere in the script.

Author: Nicholas Chammas <nicholas.chammas@gmail.com>
Author: nchammas <nicholas.chammas@gmail.com>

Closes apache#1043 from nchammas/master and squashes the following commits:

69f6e22 [Nicholas Chammas] PEP8 fixes
2627247 [Nicholas Chammas] broke up lines before they hit 100 chars
6544b7e [Nicholas Chammas] [SPARK-2065] give launched instances names
69da6cf [nchammas] Merge pull request #1 from apache/master

ec2/spark_ec2.py

@@ -200,6 +200,7 @@ def get_spark_shark_version(opts):
         sys.exit(1)
     return (version, spark_shark_map[version])
 
+
 # Attempt to resolve an appropriate AMI given the architecture and
 # region of the request.
 def get_spark_ami(opts):
@@ -418,6 +419,16 @@ def launch_cluster(conn, opts, cluster_name):
         master_nodes = master_res.instances
         print "Launched master in %s, regid = %s" % (zone, master_res.id)
 
+    # Give the instances descriptive names
+    for master in master_nodes:
+        master.add_tag(
+            key='Name',
+            value='spark-{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))
+
     # Return all the instances
     return (master_nodes, slave_nodes)