puppet: Default go-camo to listening on localhost for standalone deploys.

The default in the previous commit, inherited from camo, was to bind
to 0.0.0.0:9292.  In standalone deployments, camo is deployed on the
same host as the nginx reverse proxy, and as such there is no need to
open it up to other IPs.

Make `zulip::camo` take an optional parameter, which allows overriding
it in puppet, but skips a `zulip.conf` setting for it, since it is
unlikely to be adjust by most users.

puppet/zulip_ops/manifests/camo.pp

@@ -1,5 +1,7 @@
 class zulip_ops::camo {
-  include zulip::camo
+  class { 'zulip::camo':
+    listen_address => '0.0.0.0',
+  }
 
   zulip_ops::firewall_allow { 'camo': port => '9292' }
 }