worker-script: Default to 1 concurrent run
worker-script: Default to 1 concurrent run

1 seems to be the best setting in production, so default to that rather
than having to use --concurrent-runs=1 for each worker install.

Signed-off-by: Andy Doan <09032574f02f3d5e4e489a718b8705f28c474e94@linaro.org>

linaro-technologies/jobserv,linaro-technologies/jobserv