Cells: Add periodic instance healing

Adds a periodic task that'll sync instance information to API cells
periodically.  Does a handful of instances per periodic task run based
on config values.  Instances picked for syncing are randomized so that
multiple nova-cells services won't be syncing the same instances at
nearly the same time.

instance_updated_at_threshold -- Number of seconds after an instance was
                                 updated or deleted to continue to sync
                                 (Ie, don't sync instances updated more
                                 than 'x' seconds ago.)
instance_update_num_instances -- Number of instances to update per periodic
                                 task run

Implements blueprint nova-compute-cells

DocImpact

Change-Id: I3103c3a69ab9cf0ec3e399abe046ed0d216234ab
Merge "Cells: Add periodic instance healing"