[Feature] Allow non-pv MariaDB environment about mariadb-operator

[mmontes11]

Hey there @outductor ! Thanks for bringing this up.

I agree, this has a lot of use cases for running in dev and test scenarios, like running inside a CI pipeline where the data durability is not a concern.

As in #323, I suggest we create a flag for this:

apiVersion: mariadb.mmontes.io/v1alpha1
kind: MariaDB
metadata:
  name: mariadb
spec:
  ephemeralStorage: true

The volumeClaimTemplate will be optional now, but if neither spec.volumeClaimTemplate nor spec.ephemeralStorage are set, the validation webhook will return an error. This is to avoid situations where the user probably haven't thought about the storage too much, preventing unwanted situations where the MariaDB is running with ephemeral storage in productive environments.