Currently when jobs use semaphores they first get and lock the build
nodes and then aquire the semaphore. If there are many jobs waiting
for the semaphore this can block a substantial part of the available
resources. In order to make this safe default to acquire the semaphore
before requesting the nodes.
However in some cases when jobs with a semaphore shall run as fast as
possible in a consecutive manner then it might be preferrable to
accept some waste of resources. In order to support this use case the
job using a semaphore can override this behavior and still acquire the
semaphore after getting the nodes.
Change-Id: Id6f582ec29219d280d05319d1b822c7934437b7a
ae887dab58