Make nova-next multinode and drop tempest-slow-py3

The tempest-slow-py3 job is, well, very slow. It takes
over 2.5 hours sometimes to complete. There are a few
reasons beyond it just running slow tests but it runs
all slow tests serially and for nova it's testing things
we don't care about like network scenario tests like
test_slaac_from_os. The one benefit we get from running
tempest-slow-py3 is that it's multinode and there are
certain slow test scenarios for multinode that are
important for test coverage.

This change drops the tempest-slow-py3 job from our
job list and changes nova-next to be multinode. The
nova-next job runs a select set of tempest compute API
and scenario tests only and runs them concurrently, which
in the gate is 4 workers at a time. The nova-next job
will take a bit longer since we have to setup the subnode
now but overall it should still be faster than the
tempest-slow-py3 job and we'll save on one more node
required from nodepool to run jobs against nova changes.

The USE_PYTHON3 variable can be dropped from the nova-next
job definition now that it extends tempest-multinode-full-py3.

Depends-On: https://review.opendev.org/690469/

Change-Id: I1b7d71e833bf0743f22d7fa0241c9d1bbcd0faac

.zuul.yaml

@@ -186,7 +186,7 @@
 
 - job:
     name: nova-next
-    parent: devstack-tempest
+    parent: tempest-multinode-full-py3
     description: |
       This job was added in Newton when placement and cellsv2
       were optional. Placement and cellsv2 are required starting in Ocata. In
@@ -198,6 +198,7 @@
       Starting in Stein, the job was changed to run with python 3 and enabled
       volume multi-attach testing.
       Starting in Train, the job enabled counting quota usage from placement.
+      Starting in Ussuri, the job was changed to multinode.
       Runs all tempest compute API and most scenario tests concurrently.
     irrelevant-files: *dsvm-irrelevant-files
     # Run post-tempest tests like for nova-manage commands.
@@ -241,8 +242,6 @@
         # the tls-proxy service to be enabled. Added in Queens.
         NOVA_CONSOLE_PROXY_COMPUTE_TLS: True
         # Added in Stein.
-        USE_PYTHON3: True
-        # Added in Stein.
         ENABLE_VOLUME_MULTIATTACH: True
       devstack_services:
         tls-proxy: true
@@ -254,6 +253,14 @@
       devstack_plugins:
         # Needed for QoS port heal allocation testing.
         neutron: https://opendev.org/openstack/neutron
+    group-vars:
+      subnode:
+        devstack_localrc:
+          NOVA_USE_SERVICE_TOKEN: True
+          NOVA_CONSOLE_PROXY_COMPUTE_TLS: True
+        devstack_services:
+          tls-proxy: true
+          c-bak: false
 
 - job:
     name: nova-tempest-v2-api
@@ -406,8 +413,6 @@
         - nova-tox-functional-py36
         - tempest-integrated-compute:
             irrelevant-files: *dsvm-irrelevant-files
-        - tempest-slow-py3:
-            irrelevant-files: *dsvm-irrelevant-files
         - grenade-py3:
             irrelevant-files: *dsvm-irrelevant-files
         - tempest-ipv6-only:
@@ -422,8 +427,6 @@
         - nova-next
         - tempest-integrated-compute:
             irrelevant-files: *dsvm-irrelevant-files
-        - tempest-slow-py3:
-            irrelevant-files: *dsvm-irrelevant-files
         - grenade-py3:
             irrelevant-files: *dsvm-irrelevant-files
         - tempest-ipv6-only: