Optional size parameter for volume creation

This patch makes optional the parameter size
for creating a volume from a snapshot or other
volume.
The parameter is still required for other
kind of volume creation.

Change-Id: I75dcf0e647f6e82a2407bcf23ef359f5f87323e8
Closes-Bug: #1304463
Add commands for managing and unmanaging volumes

Add manage and unmanage commands.

Cinder code: https://review.openstack.org/#/c/72501

See also adding support for "bootable" flag for volume manage,
and changing the LVM backend to use the source-volume-name key
as part of the existing-ref structure, to make the CLI easier
to use:
https://review.openstack.org/#/c/108488/

Implements: blueprint add-export-import-volumes

Change-Id: I27d0d3396d80063a51b0fe56d2d3c92931fa9c6c
Merge "Optional size parameter for volume creation"
Merge "Add commands for managing and unmanaging volumes"