Fix yaml warning in release notes and enable yamllint strict mode

Fix yaml warning in release notes. And this patch wants to
make sure pep8 jenkins job can cover yaml warning check
in the future. So use yamllint with strict mode[1]
to treat warning as error.

[1] http://yamllint.readthedocs.io/en/stable/configuration.html#errors-and-warnings

Closes-Bug: #1778389

Change-Id: I05af5204c4d5388daa8bd3c8582fd1058df15920

releasenotes/notes/add-congress-dashboard-e302d9aeecdc0eaa.yaml

@@ -1,2 +1,3 @@
+---
 features:
   - Add congress-dashboard to horizon image.

tox.ini

@@ -41,7 +41,7 @@ commands =
   {toxinidir}/tools/validate-all-dockerfiles.sh
   python {toxinidir}/tools/validate-all-file.py
   bandit -r docker kolla tests tools
-  yamllint .
+  yamllint -s .
 
 [testenv:bandit]
 commands = bandit -r docker kolla tests tools