havana: nova compute Cells: Add filtering and weight support

[Tom Fifield]

https://review.openstack.org/#/c/16221/ fixed the cell-level scheduling so that it is no longer just random!

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.

[Anne Gentle (annegentle)]

https://review.openstack.org/#/c/44127/

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/51505
Committed: http://github.com/openstack/openstack-manuals/commit/0ce083b38b2ee8cd6d41da9223d1900515a80fd9
Submitter: Jenkins
Branch: master

commit 0ce083b38b2ee8cd6d41da9223d1900515a80fd9
Author: Summer Long <email address hidden>
Date: Mon Oct 14 11:26:44 2013 +1000

    Updated cell scheduling section for Compute

    Added mute_child, scheduler_retries, and scheduler_retry_delay options. Restructured section. Updated 'nova' -> 'Compute' in Cell Configuration Options section.

    Partial-Bug: #1187290

    Change-Id: I98902d24b5d161136e9efe6c84cf776e19b48742