[stable/mariadb] Use storageClassName for mariadb.

[gyliu513]

Fixed mariadb for #1869

/cc @prydonius

[scottrigby]

In the charts meeting we said there's no reason this can't be merged. I had a question about the need for the - abstraction, but we agreed I'll open a separate discussion issue about the differences in go template between nil and "" (empty string), and if it turns out we can easily detect the difference between the two, I could open a refactoring PR for this. 👍

[mgoodness]

/lgtm