If task can be run only on master, use shortcut

Currently Jenkins to fire WorkflowJob re-calculates the ConstistentHash
for entire cluster even if there is no nodes were updated. If cluster is
big enough (>100 nodes), it becomes a problem, because MD5 is quite
expensive itself plus all this logic comes with high memory footprint.

Using the knowledge that WorkflowJob can be executed only on Jenkins master,
we can create a shortcut that does not do expensive calculation and just
returns Runnable or `null`.