Add missing prometheus exporters to Kolla/Kolla-Ansible

[berendt]

Next steps:

1. complete libvirt-exporter integration in kolla-ansible
2. finish the preparations of the openvswitch_exporter fork (https://github.com/osism/openvswitch_exporter)
3. add ovs-/ovn-exporter container image in kolla
4. add ovs-/ovn-exporter ansible role in kolla-ansible

Open review requests:

• libvirt-exporter: https://review.opendev.org/c/openstack/kolla-ansible/+/643568
• ovs-/ovn-exporter: https://review.opendev.org/c/openstack/kolla/+/762986

[berendt]

A versioned version of the openvswitch_epxorter is now available. As first release I took v.0.0.1.

https://github.com/osism/openvswitch_exporter/releases/tag/v0.0.1

[berendt]

• [ ] Issue for the OVS exporter image: https://github.com/osism/issues/issues/26
• [ ] Issue for the OVS exporter role: https://github.com/osism/issues/issues/27

[berendt]

Tracked via other issues.