Rename zulip::postgres_appdb to zulip::postgres_appdb_base.

The purpose of this rename is to allow us to move the postgres-related
configuration out of voyager.pp.

puppet/zulip/manifests/postgres_appdb_base.pp

@@ -1,4 +1,4 @@
-class zulip::postgres_appdb {
+class zulip::postgres_appdb_base {
   include zulip::postgres_common
   include zulip::supervisor
 

puppet/zulip/manifests/voyager.pp

@@ -1,7 +1,7 @@
 class zulip::voyager {
   include zulip::base
   include zulip::app_frontend
-  include zulip::postgres_appdb
+  include zulip::postgres_appdb_base
   include zulip::redis
 
   apt::source {'zulip':

puppet/zulip_internal/manifests/postgres_appdb.pp

@@ -1,6 +1,6 @@
 class zulip_internal::postgres_appdb {
   include zulip_internal::postgres_common
-  include zulip::postgres_appdb
+  include zulip::postgres_appdb_base
 
   file { "/etc/postgresql/9.1/main/pg_hba.conf":
     require => Package["postgresql-9.1"],