Add set-bootable command

Bootable Status is set to "True" automatically when user
create a volume from a image. But user have to set bootable
status manually when creating a bootable volume manually.

blueprint add-bootable-option

this commit is related to https://review.openstack.org/#/c/84057/

Change-Id: Ib4a44334001b7b9e3b896c2e44607c414e75e952
Merge "Add set-bootable command"