Add tempest tests for DVR router state management

Add positive and negative tempest tests that assert correct operation of
a DVR router when the extension 'router-admin-state-down-before-update'
is enabled.
Added extension to neutron-tempest-plugin job.

Depends-On: https://review.opendev.org/#/c/625134/
Change-Id: Iaf24afa3d0fc28f2bec7be1b705a8d8b5ff886f8
Signed-off-by: Matt Welch <matt.welch@intel.com>
diff --git a/.zuul.yaml b/.zuul.yaml
index 4e31ba8..f5343d2 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -61,6 +61,7 @@
         - rbac-policies
         - rbac-security-groups
         - router
+        - router-admin-state-down-before-update
         - router_availability_zone
         - security-group
         - segment