hydra-queue-runner: don't clutter the system log with debug message

Avoid the frequently printed

  hydra-queue-runner[10293]: system type `x86_64-linux': 2 active, 2 allowed, starting 0 builds

message. That information is only interesting when some build are
actually started.
Merge pull request #85 from peti/dont-clutter-system-log-with-debug-messages

hydra-queue-runner: don't clutter the system log with debug message