[incubator/cassandra] change backup image, add parallel to values

[maorfr]

What this PR does / why we need it:

Change the image to the official image by our company
Allow control over level of parallelism in backup process

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: maorfr
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We suggest the following additional approver: maver1ck

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[k8s-ci-robot]

Hi @maorfr. Thanks for your PR.

I'm waiting for a helm member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[maorfr]

/assign @maver1ck
Thanks!

[maver1ck]

/ok-to-test

[maver1ck]

Don't we want to maintain default ?

[maorfr]

I always feel that the values.yaml file should provide an easy interface to change things when you need, instead of digging in the templates...
when increasing parallelism - it increases resource usage, so I think it is better left for the user to decide.

What do you think?