Use juju 3.1 for CI tests

Update the snaps to use in functional tests:
juju snap to 3.1 and microk8s snap to 1.26-strict.

Update zaza requirement to use libjuju-3.1 branch.

Change-Id: I6e251140c6426b9d58c6ca7302f395c275bbf99d

.zuul.yaml

@@ -5,3 +5,7 @@
       - microk8s-func-test
     vars:
       charm_build_name: ovn-central-k8s
+      juju_channel: 3.1/stable
+      juju_classic_mode: false
+      microk8s_channel: 1.26-strict/stable
+      microk8s_classic_mode: false

tox.ini

@@ -24,6 +24,7 @@ setenv =
   PYTHONPATH = {toxinidir}:{[vars]lib_path}:{[vars]src_path}
 passenv =
   PYTHONPATH
+  HOME
 install_command =
   pip install {opts} {packages}
 commands = stestr run --slowest {posargs}
@@ -116,7 +117,7 @@ commands =
 [testenv:func-noop]
 basepython = python3
 deps =
-    git+https://github.com/openstack-charmers/zaza.git#egg=zaza
+    git+https://github.com/openstack-charmers/zaza.git@libjuju-3.1#egg=zaza
     git+https://github.com/openstack-charmers/zaza-openstack-tests.git#egg=zaza.openstack
     git+https://opendev.org/openstack/tempest.git#egg=tempest
 commands =