store/sqlstore cleanup and postgres tests (#7595)

* sqlstore cleanup / postgres tests

* remove stopped containers

* cmd/platform compile fix

* remove test-postgres target from makefile

store/sqlstore/preference_store.go

@@ -101,7 +101,7 @@ func (s SqlPreferenceStore) save(transaction *gorp.Transaction, preference *mode
 		"Value":    preference.Value,
 	}
 
-	if *utils.Cfg.SqlSettings.DriverName == model.DATABASE_DRIVER_MYSQL {
+	if s.DriverName() == model.DATABASE_DRIVER_MYSQL {
 		if _, err := transaction.Exec(
 			`INSERT INTO
 				Preferences
@@ -112,7 +112,7 @@ func (s SqlPreferenceStore) save(transaction *gorp.Transaction, preference *mode
 				Value = :Value`, params); err != nil {
 			result.Err = model.NewAppError("SqlPreferenceStore.save", "store.sql_preference.save.updating.app_error", nil, err.Error(), http.StatusInternalServerError)
 		}
-	} else if *utils.Cfg.SqlSettings.DriverName == model.DATABASE_DRIVER_POSTGRES {
+	} else if s.DriverName() == model.DATABASE_DRIVER_POSTGRES {
 		// postgres has no way to upsert values until version 9.5 and trying inserting and then updating causes transactions to abort
 		count, err := transaction.SelectInt(
 			`SELECT