[meta] add auth-token-credential-id entry for slack (#707)

This commit adds a new auth-token-credential-id entry in addition of auth-token-id for slack token.

This is due to a bug in JJB which requires having both entries to prevent having an empty authTokenCredentialId in job config.xml (https://storyboard.openstack.org/#!/story/2007953).

Author: jmlrt

.ci/jobs/elastic+ansible-elasticsearch+master.yml

@@ -31,3 +31,4 @@
         room: infra-release-notify
         team-domain: elastic
         auth-token-id: release-slack-integration-token
+        auth-token-credential-id: release-slack-integration-token