This adds filtering and weighing support to the cells scheduler.
Adds the following config options to the 'cells' group:
scheduler_filter_classes -- list of filter classes
scheduler_weight_classes -- list of weight classes
Adds a couple of weighing modules as defaults (which removes the random
cell selection):
ram_by_instance_type: Select cells with the most capacity for the
instance type being requested.
weight_offset: Allows modifying the DB to weight a particular cell
(useful for disabling a cell)
Adds a filter class (TargetCellFilter) that allows specifying a scheduler
hint to direct a build to a particular cell if you're an admin.
DocImpact
Implements blueprint cells-filter-scheduler
Change-Id: I027c5734e3d6134127fcd4dd28b8ff39047416dc
7a5ed3e767