Switch nodepool test to containers

The nodepool-functional-container-openstack-siblings builds nodepool
container images (with openstacksdk and other "siblings" of nodepool
from source) and runs the build/boot tests with that.

The container builds of these tools are what OpenDev uses in
production, so this is more practical testing than installing by parts
on a bare Bionic system.  This allows us to remove the job duplication
in nodepool.

Backport note: this backport uses centos-8 stream job, which was
introduced with 97ccea4683.  This should
allow us to remove the non-container versions of these jobs from
nodepool tree

Cherry-picked from 5db7d5573a
Needed-By: https://review.opendev.org/c/zuul/nodepool/+/788406
Change-Id: I91345ebf509238ca1f800cb595818c023cc56b0d

.zuul.yaml

@@ -385,7 +385,9 @@
       - release-notes-jobs-python3
     check:
       jobs:
-        - nodepool-functional-openstack-src
+        - opendev-buildset-registry
+        - nodepool-build-image-siblings
+        - dib-nodepool-functional-openstack-centos-8-stream-src
         - openstacksdk-functional-devstack
         - openstacksdk-functional-devstack-networking
         - openstacksdk-functional-devstack-senlin
@@ -412,7 +414,9 @@
             voting: false
     gate:
       jobs:
-        - nodepool-functional-openstack-src
+        - opendev-buildset-registry
+        - nodepool-build-image-siblings
+        - dib-nodepool-functional-openstack-centos-8-stream-src
         - openstacksdk-functional-devstack
         - openstacksdk-functional-devstack-networking
         - openstacksdk-functional-devstack-senlin