support for running a bot/pre-deploy.sh script

[trz42]

This is to ensure that the deployment can only begin if necessary conditions are met. Such conditions could be that builds for all architectures have succeeded, all jobs have finished, etc.

The actual deployment code may run that script in a separate job and only proceed if the script exits with return value 0.

Open question is how to handle cases when builds for some architectures don't succeed to build. Maybe this script should be accompanied with a new bot command bot: deploy FILTERS which then doesn't use that script (or at least not if any FILTER applies).