Throttle Docker Builds to 1 Per-Node

Throttle Docker Builds to 1 Per-Node

Docker builds are currently timing out when multiple builds are running
on the same node. Builds start in parallel but Docker only allows one
to run at a time. By throttling builds to 1 per-node Docker builds will
run sequentially, as they are currently, but not timeout.

JIRA: RELENG-315

Change-Id: Ib64252f9449cccdc35856debafd3bcb8796f1100
Signed-off-by: Trevor Bramwell <tbramwell@linuxfoundation.org>