Make neutron-tempest-plugin-scenario-linuxbridge job gating

This job works as voting in check queue since [1]
was merged.
It works without problems for few weeks so now it's
time to add it to gate queue also.

[1] https://review.openstack.org/#/c/552689/

Change-Id: Ib0d46d93db0a9098a9cd9321b5a7bee1017e52fb
diff --git a/.zuul.yaml b/.zuul.yaml
index 636327e..a854586 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -208,6 +208,7 @@
     gate:
       jobs:
         - neutron-tempest-plugin-api
+        - neutron-tempest-plugin-scenario-linuxbridge
         - build-openstack-sphinx-docs
 
 - project-template: