Add volume mount to the git sync container so that certificates can be mounted

[PaulienVa]

Description

When using kubernetes executors, volume mounts are not mounted to the gitsync initContainer. This causes a problem when a certificate is needed to access a git repository (also when using https:// instead of ssh).
The webserver and the scheduler are able to sync the repositories, to avoid an inconsistent state of the operator, the executors should do to.

Definition of Done Checklist

• Not all of these items are applicable to all PRs, the author should update this template to only leave the boxes in that are relevant
• Please make sure all these things are done and tick the boxes

Author

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1. Changes are OpenShift compatible
2. CRD changes approved
3. CRD documentation for all fields, following the style guide.
4. Helm chart can be installed and deployed operator works
5. Integration tests passed (for non trivial changes)
6. Changes need to be "offline" compatible

Reviewer

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1. Code contains useful comments
2. Code contains useful logging statements
3. (Integration-)Test cases added
4. Documentation added or updated. Follows the style guide.
5. Changelog updated
6. Cargo.toml only contains references to git tags (not specific commits or branches)

Acceptance

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1. Feature Tracker has been updated
2. Proper release label has been added
3. Roadmap has been updated

[PaulienVa]

#456

Proper fix is implemented in PR above