Fix dogtag functional job

Use the correct service name so that dogtag will actually get installed.

Drop the mozilla-nss-devel pkg which no longer exists for Fedora 31.

TODO: The job will still fail because devstack replaces the system
python3-six pkg with some updated version, but pkispawn runs with
"python3 -I" which ignores user-installed libs, causing it to fail
with a "Cannot import six" error.

Change-Id: I6cdab2d58f47650f296f8e79ee718647c3160142

.zuul.yaml

@@ -20,7 +20,7 @@
       Functional testing for DogTag backend
     vars:
       devstack_services:
-        dogtag: true
+        barbican-dogtag: true
 
 - job:
     name: barbican-vault-tox-functional

bindep.txt

@@ -1,7 +1,6 @@
 # See openstack-infra/project-config:jenkins/data/bindep-fallback.txt
 # This is used by bindep: sudo [apt-get | yum] install $(bindep -b)
 
-mozilla-nss-devel [platform:rpm]
 nss-devel [platform:rpm]
 libnss3-dev [platform:dpkg]