Send Slack notifications for Jenkins jobs

Environment variables

JENKINS_BASE_URL='https://jenkins.example.com/'
JENKINS_USER='user.name@example.com'
JENKINS_TOKEN='xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx'
SLACK_TOKEN='xoxp-xxxxxxxxxx-xxxxxxxxxxxx-xxxxxxxxxxxx-xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx'
SLACK_WEBHOOK='https://example.slack.com/services/hooks/hook-name?token=xxxxxxxxxxxxxxxxxxxxxxxx'