Move horizon test from tempest-horizon to tempest

As disscussed in Wallaby PTG[1], QA and Horizon team
decided to move the horizon dashboard test from tempest-horizon
to Tempest. As next step, we can remove the tempest-horizon
plugin which will ease the maintaince of horizon tempest test.

[1] https://etherpad.opendev.org/p/qa-wallaby-ptg

Change-Id: Id2ced856a41548a0b49e594ee5fed6ed28785f24
diff --git a/releasenotes/notes/merge-tempest-horizon-plugin-39d555339ab8c7ce.yaml b/releasenotes/notes/merge-tempest-horizon-plugin-39d555339ab8c7ce.yaml
new file mode 100644
index 0000000..ff406fb
--- /dev/null
+++ b/releasenotes/notes/merge-tempest-horizon-plugin-39d555339ab8c7ce.yaml
@@ -0,0 +1,6 @@
+---
+prelude: >
+    The integrated horizon dashboard test is now moved
+    from tempest-horizon plugin into Tempest. You do not need
+    to install tempest-horizon to run the horizon test which
+    can be run using Tempest itself.