gitlab/database.yml: Tweak to integrate gitlab with internal postgresql

We tweak database.yml to point to our postgresql unix socket; set
adapter to hardcoded postgresql, encoding to unicode and omit collation
(which according to omnibus-gitlab is used for mysql only).

The only instance parameter imported from omnibus is `db_pool` - how
many connection to a DB to keep open in a RoR thread/process.

XXX we use db's superuser as a user to connect. Is it ok to do even if
    the whole DB is used only for gitlab? (I think it is ok for the
    first iteration, but we'll probably need to refine this later)

/cc @kazuhiko, @jerome

software/gitlab/gitlab-parameters.cfg

@@ -9,3 +9,6 @@
 # (last updated for omnibus-gitlab 8.2.3+ce.0-0-g8eda093)
 
 [gitlab-parameters]
+
+# db advanced
+configuration.db_pool                   = 10

software/gitlab/instance-gitlab.cfg.in

@@ -147,6 +147,8 @@ template = {{ config_ru_in }}
 [database.yml]
 <= gitlab-etc-template
 template= {{ database_yml_in }}
+context-extra =
+    section pgsql                   service-postgresql
 
 [gitlab-shell-config.yml]
 <= etc-template

software/gitlab/template/database.yml.in

@@ -4,16 +4,24 @@
 # https://gitlab.com/gitlab-org/omnibus-gitlab/blob/master/files/gitlab-cookbooks/gitlab/templates/default/database.yml.erb
 # (last updated for 8.2.3+ce.0-0-g8eda093)
 
+{% from 'macrolib.cfg.in' import cfg  with context %}
+
 production:
-  adapter: <%= @db_adapter %>
-  encoding: <%= @db_encoding %>
+  adapter: postgresql
+  encoding: unicode
+  {# collation is mainly for mysql
   collation: <%= @db_collation %>
-  database: <%= @db_database %>
-  pool: <%= @db_pool %>
-  username: <%= single_quote(@db_username) %>
-  password: <%= single_quote(@db_password) %>
-  host: <%= single_quote(@db_host) %>
-  port: <%= @db_port %>
-  socket: <%= single_quote(@db_socket) %>
+  #}
+  database: {{ pgsql.dbname }}
+  pool: {{ cfg('db_pool') }}
+  {# XXX is it ok to use superuser, even if the whole database is only for gitlab? #}
+  username: '{{ pgsql.superuser }}'
+  {# we have no password - access is via unix socket #}
+  password:
+  host:     '{{ pgsql["pgdata-directory"] }}'
+  port:
+  socket:
+  {# not needed for unix socket
   sslmode: <%= single_quote(@db_sslmode) %>
   sslrootcert: <%= single_quote(@db_sslrootcert) %>
+  #}