[DNM] Configure nfs server with 2 backends for tests

[cescgina]

Configure two different backends with different backend_volume_name to
be able to run migration tests and check if they will work without
assuming that multiple pools are placed within the same cinder backend.

Depends-On: https://review.opendev.org/c/openstack/watcher/+/963857
Depends-On: https://review.opendev.org/c/openstack/watcher-tempest-plugin/+/963860

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[openshift-ci]

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[softwarefactory-project-zuul]

This change depends on a change that failed to merge.

Change https://review.opendev.org/c/openstack/watcher-tempest-plugin/+/963860 is needed.

[cescgina]

recheck

[softwarefactory-project-zuul]

Build failed (check pipeline). Post recheck (without leading slash)
to rerun all jobs. Make sure the failure cause has been resolved before
you rerun jobs.

https://softwarefactory-project.io/zuul/t/rdoproject.org/buildset/d6ef5513d28547d4ae3d9c69dfbc5016

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