Use and validate database type before connecting

Just the start of changes needed for T529.

app.go

@@ -403,8 +403,13 @@ func Serve() {
 }
 
 func connectToDatabase(app *app) {
-	log.Info("Connecting to database...")
-	db, err := sql.Open("mysql", fmt.Sprintf("%s:%s@tcp(%s:%d)/%s?charset=utf8mb4&parseTime=true&loc=%s", app.cfg.Database.User, app.cfg.Database.Password, app.cfg.Database.Host, app.cfg.Database.Port, app.cfg.Database.Database, url.QueryEscape(time.Local.String())))
+	if app.cfg.Database.Type != "mysql" {
+		log.Error("Invalid database type '%s'. Only 'mysql' is supported right now.", app.cfg.Database.Type)
+		os.Exit(1)
+	}
+
+	log.Info("Connecting to %s database...", app.cfg.Database.Type)
+	db, err := sql.Open(app.cfg.Database.Type, fmt.Sprintf("%s:%s@tcp(%s:%d)/%s?charset=utf8mb4&parseTime=true&loc=%s", app.cfg.Database.User, app.cfg.Database.Password, app.cfg.Database.Host, app.cfg.Database.Port, app.cfg.Database.Database, url.QueryEscape(time.Local.String())))
 	if err != nil {
 		log.Error("%s", err)
 		os.Exit(1)