Add a relative_priority field to node requests and continuously
adjust it for each queue item based on the contents of queues.
This allows for a more fair distribution of build resources between
different projects. The first item in a pipeline from a given
project (or, in the case of a dependent pipeline, group of projects)
has equal priority to all other first-items of other projcets in
the same pipeline. Second items have a lower priority, etc.
Depends-On: https://review.openstack.org/620954
Change-Id: Id3799aeb2cec6d96a662bfa394a538050f7ea947
James E. Blair