adding option to make volume optional

[beatkind]

Description of the change

This PR adds the option to make the Persistante Volume optional.

Benefits

Persistency is optional.

Possible drawbacks

When Upgrading without reading change notes, the PV will not be mounted any more if the newly introduced value is not set.

Applicable issues

• fixes #17

Additional Infos

Should be merged after #23

Checklist

• [X] Chart version bumped in Chart.yaml according to semver.
• [X] Variables are documented in the README.md

[beatkind]

@dirsigler fixed the linting and resolved the conflict, ready to merge now