doc: Updates to federated config.

Reverted Emperor versionadded to Dumpling as it gets backported.
Added default index and bucket pools to pool creation
Added default default_placment setting
Added placement_pools key val pair examples.
Added comments for re-running the procedure for the secondary region.

Signed-off-by: John Wilkins <john.wilkins@inktank.com>
doc: Added additional comments on placement targets and default placement.

Signed-off-by: John Wilkins <john.wilkins@inktank.com>