[goal] Migrate watcher-tempest-plugin jobs to focal

As per victoria cycle testing runtime and community goal[1]
we need to migrate upstream CI/CD to Ubuntu Focal(20.04).

Most of the Tempest jobs will be migrate automatically once devstack
base job start running on Focal(Depends-On).

Stable jobs testing stable branch needs to keep running on their supported
distro version which is bionic from stein till ussuri.


[1] https://governance.openstack.org/tc/goals/selected/victoria/migrate-ci-cd-jobs-to-ubuntu-focal.html

Change-Id: I175b41351f4f34ee2a4916dda2f610bc7716d9c3
Story: #2007865
Task: #40227

.zuul.yaml

@@ -19,11 +19,13 @@
 - job:
     name: watcher-tempest-functional-ussuri
     parent: watcher-tempest-functional
+    nodeset: openstack-single-node-bionic
     override-checkout: stable/ussuri
 
 - job:
     name: watcher-tempest-functional-train
     parent: watcher-tempest-functional
+    nodeset: openstack-single-node-bionic
     override-checkout: stable/train
     vars:
       devstack_localrc:
@@ -32,6 +34,7 @@
 - job:
     name: watcher-tempest-functional-stein
     parent: watcher-tempest-functional
+    nodeset: openstack-single-node-bionic
     override-checkout: stable/stein
     vars:
       devstack_localrc:

test-requirements.txt

@@ -3,6 +3,10 @@
 # process, which may cause wedges in the gate later.
 
 hacking<0.13,>=0.12.0 # Apache-2.0
+# remove this pyflakes from here once you bump the
+# hacking to 3.2.0 or above. hacking 3.2.0 takes
+# care of pyflakes version compatibilty.
+pyflakes>=2.1.1
 
 coverage!=4.4,>=4.0 # Apache-2.0
 doc8>=0.6.0 # Apache-2.0