Enable libvirt discard support

[Edward Hope-Morley]

In order for guest instances to be able to issue discard requests against their backing storage Nova needs to enable discard=unmap in libvirt (added back in the Juno release [1]). If we enable this by default in the nova-compute charm it will allow users to optionally enable and use discard/trim in their instances (as long as they are created using the correct properties from glance i.e. hw_disk_bus=scsi and hw_scsi_model=virtio-scsi hw_disk_bus=scsi). Note that this is limited to Nova storage i.e. cinder attached volumes will need the same functionality enabled in Cinder to allow discard to be enabled for attached volumes [2][3].

[1] https://specs.openstack.org/openstack/nova-specs/specs/juno/implemented/libvirt-disk-discard-option.html
[2] https://specs.openstack.org/openstack/cinder-specs/specs/liberty/cinder-backend-report-discard.html
[3] https://specs.openstack.org/openstack/nova-specs/specs/mitaka/implemented/cinder-backend-report-discard.html

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/582134

[Edward Hope-Morley (hopem)]

Adding cinder so that we can enable there also.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/582134
Committed: https://git.openstack.org/cgit/openstack/charm-nova-compute/commit/?id=2f9bbed3a9a3b1a541124f484ae324a809262fb5
Submitter: Zuul
Branch: master

commit 2f9bbed3a9a3b1a541124f484ae324a809262fb5
Author: Edward Hope-Morley <email address hidden>
Date: Thu Jul 12 11:27:22 2018 +0100

    Enable discard

    By default discard/trim requests are disabled for libvirt. This
    patch enables them by default although in order to actually use
    them, instances still need be using scsi disk devices. Also this
    patch only permits Nova storage (e.g. instance root disk) to use
    discard. For Cinder volumes this must be enabled in Cinder.

    Change-Id: Iceb22fe5206629bc7b4096f518b8977c2e14def3
    Closes-Bug: #1781382

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/605654

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/605654
Committed: https://git.openstack.org/cgit/openstack/charm-cinder-ceph/commit/?id=9444b4402cfd3c38d737b2bcdda4fd883319d21a
Submitter: Zuul
Branch: master

commit 9444b4402cfd3c38d737b2bcdda4fd883319d21a
Author: Edward Hope-Morley <email address hidden>
Date: Thu Sep 27 12:10:03 2018 +0100

    Enable discard

    By default nova/libvirt will not enable trim for
    attached volumes so to allow users to use this
    feature we now enable it by default.

    Also removed < Icehouse unit test.

    Change-Id: I58ffaa43e2836068aeed7795df670d279d5e28f8
    Closes-Bug: #1781382