py3: Explicitly keep our wal-e PostgreSQL replication on Python 2.

On `trusty` there is no package for `boto` or `gevent` on Python 3, both
of which are dependencies of `wal-e` (at the version we've pinned.)  This
is something used only on database servers and only in a replication
scenario, and it doesn't involve any of our code outside the wal-e repo,
so the Python version it uses is quite independent of the Zulip
application server itself and the rest of our code.  For now, keep it
explicitly on Python 2 while we move forward for most everything else.

puppet/zulip_ops/manifests/postgres_common.pp

@@ -14,11 +14,11 @@ class zulip_ops::postgres_common {
   package { $internal_postgres_packages: ensure => "installed" }
 
   exec {"pip_wal-e":
-    command  => "/usr/bin/pip install git+git://github.com/zbenjamin/wal-e.git#egg=wal-e",
+    # On trusty, there is no python3-boto or python3-gevent package,
+    # so we keep our `wal-e` explicitly on Python 2 for now.
+    command  => "/usr/bin/pip2 install git+git://github.com/zbenjamin/wal-e.git#egg=wal-e",
     creates  => "/usr/local/bin/wal-e",
-    require  => Package['python3-pip',
-                        # 'python3-boto', 'python3-gevent', # missing on trusty
-                        'python-pip', 'python-boto', 'python-gevent',
+    require  => Package['python-pip', 'python-boto', 'python-gevent',
                         'lzop', 'pv'],
   }