Cells: Add filtering and weight support

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
Imported Translations from Transifex

Change-Id: I47a6645025e6a74a7205d1eadda3cb36295d341b
Merge "Cells: Add filtering and weight support"