Use deferrable_epp for .my.cnf to ensure root password resolve correctly

[jiayuchen888]

Summary

When the root password is type deferred, we have to defer the template rendering(.my.cnf) using the stdlib::deferrable_epp() function.

Checklist

• [ ] 🟢 Spec tests.
• [ ] 🟢 Acceptance tests.
• [x] Manually verified. (For example puppet apply)