Add ability to support discard/unmap/trim for Cinder backend

During volume attach if a cinder backend supports discard Nova should enable this. This functionality requires properties in the glance image to enable creation of the correct scsi controller on the instance.

Required glance properties are
hw_scsi_model=virtio-scsi
hw_disk_bus=scsi

Without the correct glance properties this flag will be ignored. The old scsi connector just ignores this setting.

A check is also added to make sure the minimum version of QEMU is present. If min version requirement is not satisfied the discard flag will not be added to the attached volume.

Work is required on Cinder side to make use of this functionality.

Cinder BP: https://blueprints.launchpad.net/cinder/+spec/cinder-backend-report-discard
Cinder Spec: Updated version https://review.openstack.org/#/c/211583/
Cinder Patch: https://review.openstack.org/#/c/205717/