Split off the finalization part of the volume manager's
extend_volume method and make it externally callable as the new
os-extend_volume_completion admin volume action.
This is the first part of a feature that will allow volume drivers
to rely on feedback from Nova when extending attached volumes,
allowing e.g. NFS-based drivers to support online extend.
See the linked blueprint for details.
Implements: bp extend-volume-completion-action
Change-Id: I4aaa5da1ad67a948102c498483de318bd245d86b
2a1a0bc3e2