Scale lab queue limits based on number of devices

[broonie]

Since the introduction of support for limiting the number of jobs in aeb2684 we queue on labs we've used an absolute limit on the number of jobs that's specified as a simple number of jobs. At the minute that's 50. This limit is applied no matter how many devices that a lab has available, so for example with my lab where I have 5 BeagleBone Blacks available with average job runtimes of approximately 5 minutes we'd clear 50 jobs in about an hour which is reasonably responsive. In contrast if I only had one board the same jobs would take over 4 hours which is much less usable.

A simple improvement would be to scale the limit based on the number of devices of a given type that a lab has, picking a smaller limit per board but allowing us to make use of larger lab capacities.