py3: Explicitly keep one boto-using ops script on Python 2.

This script in `zulip_ops` is handy for managing EC2 instances.  It uses
`boto`, which isn't available in `trusty` for Python 3.  The use of
`boto` here isn't particularly deep, so we could replace it with some
more manual HTTP calls if it comes to that.  For now, just mark it to
stay on Python 2 while we move the app and all the rest of the ops code
(except this and another straggler or two) to Python 3.

Also make a comment on this package in the Puppet manifest clearer
about what it specifically refers to.

puppet/zulip_ops/files/zulip-ec2-configure-interfaces

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python2.7
 # -*- coding: utf-8 -*-
 #
 # Copyright © 2013 Zulip, Inc.

puppet/zulip_ops/manifests/base.pp

@@ -19,9 +19,8 @@ class zulip_ops::base {
                         # Needed for zulip-ec2-configure-network-interfaces
                         "python3-six",
                         "python-six",
-                          # This one is needed for postgres as well
                         # "python3-boto", # missing on trusty
-                        "python-boto",
+                        "python-boto", # needed for postgres_common too
                         "python3-netifaces",
                         "python-netifaces",
                         # Popular editors