Buildslave priority

[gracinet]

In a swarm with heterogeneous servers, it can happen that a few are less or more powerful than the others.

Buildbot provides the max_builds parameter to control how many parallel builds a given buildslave can take, but that's not enough. For instance, at Anybox, we have currently a slow but highly threaded server. It won't be slower for 6 builds than for one. Still, we prefer to use it only if the quicker ones are already busy.

Buildbot allows customization of the choice by means of the nextSlave callable. We could write one using a slave_priority property.