Installer: Bootstrap optional PostgreSQL/Redis (#3152)

* Make optional postgres and redis in installer

install/config/config.yml

@@ -37,3 +37,8 @@ flags:
     disable_signup_without_invite: true
     disable_user_create_org: false
     allow_raw_resources: true
+
+{{if .IsPostgreSQL}}
+postgres:
+  connection_string: postgresql://pangolin:{{.IsPostgreSQLPass}}@postgres:5432/pangolin
+{{end}}