Adding shellcheck GitHub Action

Content of this PR:

it introduces a GitHub Action to perform shellcheck static code analysis of all the .sh files
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
it adds a missing call to trap system_failure ERR EXIT in run.sh in order to revoke the Orka token at the end of the execution

macstadium / orka-integrations