Add whitebox-devstack-multinode job to periodic

Whitebox is a tempest plugin whose scope specifically includes peaking
behind the curtain. It is used for integration tests that cannot
perform their assertions based on just the REST API, which is the
limit in Tempest proper. Whitebox allows things like examining the
instance XML and querying the database. While it is not limited to
Nova in theory, in practice at the time of this commit, it only
contains compute-related tests.

This patch adds the basic integration job that whitebox uses in its
own CI to Nova's periodic weekly pipeline. Whitebox does a lof of
tests around CPU pinning, and therefore uses nodepool labels that
provide nested KVM support. For that reason, it's considered wasteful
to run it against every single Nova patch, so periodic is being
proposed here.

As a periodic job, its status will have to be a recurring item on the
Nova team meeting, similar to how the Placement periodic job is
treated.

In addition, we also add it to the experimental pipeline to allow for
manual trigger if deemed necessary on any particular patch.

Change-Id: I0e1b754d0b94194cf9baeb6e188036b03be727db

.zuul.yaml

@@ -887,6 +887,7 @@
         - nova-emulation
         - ironic-tempest-ipa-wholedisk-direct-tinyipa-multinode-shard
         - tempest-centos9-stream-fips
+        - whitebox-devstack-multinode
     experimental:
       jobs:
         - ironic-tempest-bfv:
@@ -917,3 +918,4 @@
         - nova-emulation
         - tempest-integrated-compute-centos-9-stream:
             irrelevant-files: *nova-base-irrelevant-files
+        - whitebox-devstack-multinode