testing: Run functional tests under [testenv:cover]

Nowadays a significant amount of our test code coverage comes from
functional tests rather than just from unit tests.

Currently, we run unit tests under [testenv:cover] via .stestr.conf
and this just adds a run of the functional tests without API samples or
notification samples or database-only tests, for the sake of brevity.

In local testing this increases our Coverage Report from 87% to 89%
overall. And for the particular file I'm interested in,
nova/limit/utils.py, it increases coverage from 66% to 73%.

I'm doing this as a base for a bug fix in nova/limit/utils.py in the
next patch and with this change, I see coverage increase to 90%.

Change-Id: Iec0a9e38f3641e973894748ab2a14d1bd838e904
Signed-off-by: melanie witt <melwittt@gmail.com>

.zuul.yaml

@@ -852,6 +852,9 @@
             irrelevant-files: *nova-base-irrelevant-files
             voting: false
         - nova-tox-py312-threading
+        - openstack-tox-cover:
+            # Allow more time for unit and functional tests to run.
+            timeout: 4200
     gate:
       jobs:
         - nova-live-migration