[stable-only] Remove nova-emulation from check pipeline of Yoga

Yoga will transition soon to Unmaintained and nova-emulation job is
almost constantly failing on stable/yoga now, so this patch moves it
to experimental pipeline.
Note that nova-emulation is only running on stable/yoga, and we don't
run it on newer branches. Only periodic-weekly pipeline is targeted for
this job.

Change-Id: Iff6ffab833754cf39c47483a48a5df3b749fcffd

.zuul.yaml

@@ -661,7 +661,6 @@
         - nova-multi-cell
         - nova-next
         - nova-ovs-hybrid-plug
-        - nova-emulation
         - nova-tox-validate-backport:
             voting: false
         - nova-tox-functional-centos8-py36
@@ -742,6 +741,7 @@
         - devstack-plugin-nfs-tempest-full:
             irrelevant-files: *nova-base-irrelevant-files
         - nova-osprofiler-redis
+        - nova-emulation
         - tempest-pg-full:
             irrelevant-files: *nova-base-irrelevant-files
         - nova-tempest-full-oslo.versionedobjects: