Extend volume type with extra-specs options to be able to execute well known action during volume creation/resize

In order to do certain types of initialization actions when a new volume is created or resized we can add "volume_init_method/volume_resize_method" or "volume_format" parameter which can be used by volume manager.
Primarily this spec is about creating a general approach for [1], [2] changes.
[1] https://review.openstack.org/#/c/357989/
[2] https://review.openstack.org/#/c/362167/