INF-194 Add staging deploys to CircleCi (#3545)

New Deploy Workflow example (for feature branches) using matrix jobs and fan-out/fan-in workflows.

Add hold jobs for master:

* First job will deploy to all staging nodes running commits found in master or release-* branches.
* Next jobs can force deploy to all nodes, by service, or a specific node.

Add hold jobs for all other branches:

* Jobs will force deploy to all nodes, by service, or a specific node.
* No checking if the node is currently "reserved" by someone else. Check #eng-deploys-announcements prior to "reserving" a node.
* When on a release-* branch and all nodes have been deployed to, a job is visible to deploy to prod.
deploy-ci.py doesn't yet deploy to prod.

Follow-up PR will:

* Remove the first hold job for master.
* Change Slack channel to be default channel.