Support zun / ironic openstack services

### Dependencies

https://docs.openstack.org/zun/xena/install
https://docs.openstack.org/ironic/xena/install

### Feature Request

I would like to be able to provision from Jenkins containers or bare metal machines in addition to VMs.

VMs are great, but slow to startup and slow to run. We also have specific hardware that we would like to run our CI tests on.
I think it would be easier to expose the hardware to a container (as simple as sharing `/sys/class` with the container) than with a VM.

Since we have specific hardware, we require using a service like openstack [placement](https://docs.openstack.org/placement/xena/install/) to keep track of the harware used by the tests (we cannot run 2 tests on the same hardware at the same time, or it will cause conflicts). (so simply using docker cloud plugin on jenkins is not enough)

Is there a reason why this plugin only support [nova](https://docs.openstack.org/nova/xena/install/) (VMs) provisioning ?