Integrate gitlab-ci/delete-infrastucture and 1 more

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Commit: seco-ne/yocto/infrastructure/gitlab-ci@01d6973b

Scripts: Add script to delete all integration/ branches

It happens that the integration branches are left over from provious
runs of the pipeline, when MRs are closed without merging, for example.

Also a bug in the gitlab-ci makes the pipeline fail, when an integration
branch should be reused. The new scripts just deletes all integration
branch in the given project tree without further checks.

Needs to be run manually.

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Commit: seco-ne/yocto/infrastructure/gitlab-ci@c05acbe1

Add scripts to setup a virtual python environment

Use:
```
cd script
source setup_venv.sh
```
to change to the venv.