Adding shellcheck GitHub Action

[Lucas-C]

Content of this PR:

• it introduces a GitHub Action to perform shellcheck static code analysis of the .sh files in GitLab/scripts/. This check could be extended to analyze more .sh files of this repo later on.
• it includes fixes for all shellcheck warnings on the scripts in Gitlab/scripts/
• it adds a check that authentication on $ORKA_ENDPOINT/token is successful in prepare.sh
• it sets a ConnectTimeout of 60 seconds when performing the first SSH connection in prepare.sh

[Lucas-C]

GitHup Actions pipeline result: https://github.com/voyages-sncf-technologies/orka-integrations/runs/2810307325

[ispasov]

Thanks for the contribution. I have a few minor comments.

[Lucas-C]

Comments answered. Thanks for the review!

[Lucas-C]

Change made. I think this is ready to be merged

[ispasov]

There are still two occurrences of the long syntax - in run.sh and prepare.sh. Can you check them out, please?

[Lucas-C]

Done