[nova][CI] GPUs in the gate

Hey all,

Following up on the CI session during the PTG [1], I wanted to get the
ball rolling on getting GPU hardware into the gate somehow. Initially
the plan was to do it through OpenLab and by convincing NVIDIA to
donate the cards, but after a conversation with Sean McGinnis it
appears Infra have access to machines with GPUs.

>From Nova's POV, the requirements are:
* The machines with GPUs should probably be Ironic baremetal nodes and
not VMs [*].
* The GPUs need to support virtualization. It's hard to get a
comprehensive list of GPUs that do, but Nova's own docs [2] mention
two: Intel cards with GVT [3] and NVIDIA GRID [4].

So I think at this point the question is whether Infra can support
those reqs. If yes, we can start concrete steps towards getting those
machines used by a CI job. If not, we'll fall back to OpenLab and try
to get them hardware.

[*] Could we do double-passthrough? Could the card be passed through
to the L1 guest via the PCI passthrough mechanism, and then into the
L2 guest via the mdev mechanism?

[1] https://etherpad.openstack.org/p/nova-ptg-train-ci
[2] https://docs.openstack.org/nova/rocky/admin/virtual-gpu.html
[3] https://01.org/igvt-g
[4] https://docs.nvidia.com/grid/5.0/pdf/grid-vgpu-user-guide.pdf