concurrency check uses the postgres db to look at dag_runs table
to determine what is currently running.
update_site and deploy_site have been added/refactored to be a
linear flow, with a function as an error handler. This was done
to prevent runs of downstream error handling steps when their
parent was set to upstream failed status.
Added simple test for failure handler.
Added testing for concurrency check logic
Added requirements for testing purposes
Change-Id: I29a2509df999b4714a9ade2ebabac7522504e24e
e24c2504e0