py3: Move python-gevent dependency to narrow its scope.

This is only actually used in our `wal-e` setup, which is in
zulip_ops::postgres_common.  (In fact the only mentions of `gevent` in
our whole Git history are for `wal-e`.)  So remove where we mention it
on the broader zulip::postgres_common module, and move it where it's
needed.

This follows up on 98cef0ab4 by eliminating the only dependency
outside of the `zulip_ops` Puppet tree on a system Python-library
package which isn't available in `trusty` for Python 3.

puppet/zulip/manifests/postgres_common.pp

@@ -4,8 +4,6 @@ class zulip::postgres_common {
                         # tools for database monitoring
                         "ptop",
                         # Python modules used in our monitoring/worker threads
-                        # "python3-gevent", # missing on trusty
-                        "python-gevent",
                         "python3-tz", # TODO: use a virtualenv instead
                         "python-tz", # TODO: use a virtualenv instead
                         "python3-dateutil", # TODO: use a virtualenv instead

puppet/zulip_ops/manifests/postgres_common.pp

@@ -6,6 +6,8 @@ class zulip_ops::postgres_common {
                                  "pv",
                                  "python3-pip",
                                  "python-pip",
+                                 # "python3-gevent", # missing on trusty
+                                 "python-gevent",
                                  # Postgres Nagios check plugin
                                  "check-postgres",
                                  ]