Truncate backup-retention-cronjob name for template in influxdb chart

In `_helpers.tpl`, we have two places where we truncate names down to 63 characters. But then in the backup-retention-cronjob template, we append `-backup-retention` to the name; that's an extra 17 characters.

I suggest truncating the backup-retention-cronjob to 63 chars, we'll need to truncate it before we append the additional chars. Otherwise, the name won't make very much sense.

Or... We could provide a way to override the name in the values.yaml file?

Reference(s):
- https://github.com/influxdata/helm-charts/blob/master/charts/influxdb/templates/_helpers.tpl#L6
- https://github.com/influxdata/helm-charts/blob/master/charts/influxdb/templates/backup-retention-cronjob.yaml#L9

Start with charts/influxdb/templates/_helpers.tpl and charts/influxdb/templates/backup-retention-cronjob.yaml, following the existing name truncation and the appended -backup-retention suffix. Render the chart with a long release or chart name and verify that the resulting backup-retention-cronjob name stays within 63 characters and remains meaningful.

Written by the indexing model from the issue text.