Fix MySQL legacy username regression (#3141)

* Fix the mysql legacy username length

* Remove boolean parameter

* Add a MySQL 5.6 container to test the legacy MySQL plugin against

* Add database plugins to the make file

* Fix credsutil test

plugins/database/mysql/mysql-legacy-database-plugin/main.go

@@ -0,0 +1,21 @@
+package main
+
+import (
+	"log"
+	"os"
+
+	"github.com/hashicorp/vault/helper/pluginutil"
+	"github.com/hashicorp/vault/plugins/database/mysql"
+)
+
+func main() {
+	apiClientMeta := &pluginutil.APIClientMeta{}
+	flags := apiClientMeta.FlagSet()
+	flags.Parse(os.Args)
+
+	err := mysql.RunLegacy(apiClientMeta.GetTLSConfig())
+	if err != nil {
+		log.Println(err)
+		os.Exit(1)
+	}
+}