Query "Needs Validation" label from GitLab issues

Tails images build reproducibility is tested if the corresponding ticket has "Needs Validation" status:

• https://git.tails.boum.org/puppet-tails/tree/files/jenkins/slaves/isobuilders/decide_if_reproduce

Email notifications are sent on job status change if the ticket has "Needs Validation" status (currently disabled):

• https://git.tails.boum.org/puppet-tails/tree/files/jenkins/slaves/isobuilders/output_ISO_builds_and_tests_notifications

These are branches that track the modifications needed for this change:

• https://git.tails.boum.org/puppet-tails/log/?h=53_query-gitlab-needs-validation
• puppet-git.lizard:jenkins-jobs@53-switch-to-gitlab-apikey
• puppet-git.lizard:puppet-tails_secrets_jenkins@53_query-gitlab-needs-validation