add mariadb.global.defaultStorageClass to values.yaml

[jessebot]

Description of the change

Add mariadb.global.defaultStorageClass to our values.yaml and clean up the README to also include mariadb.primary.persistence.storageClass and mariadb.primary.persistence.existingClaim as they'd be common parameters.

Benefits

Less confusion about how to use the MariaDB subchart because it better reflects the bitnami helm chart.

Possible drawbacks

none that I can think of, but always happy for feedback :)

Applicable issues

• fixes mariadb persisted replicated pvc doesn't use the same storage class #123

Additional information

nope

Checklist

• I have read the CONTRIBUTING.md doc.
• DCO has been signed off on the commit.
• Chart version bumped in Chart.yaml according to semver.
• (optional) Parameters are documented in the README.md