helm: add image pull secrets on ServiceAccount for manager and scylla cluster

Needed to be able to download images from docker registry with authentication Also update scylla-operator to use same configuration instead of specific one on sts

Description of your changes:

Which issue is resolved by this Pull Request: Resolves #

Checklist:

• [ ] Documentation has been updated, if necessary.
• [ ] Image has been built (make docker-build) on the last commit.