Fix propose-project-config-update

After the recent change, the job now fails since ruamel.yaml is not
installed. Change the job to use tox to install dependency.

This creates a new tox environment that is then called from the proposal
script.

Failure log:
http://logs.openstack.org/periodic/git.openstack.org/openstack-infra/project-config/master/propose-project-config-update/bf68ac6/

Change-Id: I02ce33f10d388531806380f464dacc5617c74f1f

tools/propose_projects_yaml_update.sh

@@ -52,7 +52,7 @@ else
 fi
 
 git review -s
-python tools/normalize_projects_yaml.py
+tox -e normalize-projects-yaml
 
 if ! git diff --stat --exit-code HEAD ; then
     # Commit and review

tox.ini

@@ -133,3 +133,9 @@ deps =
 commands =
   python tools/add-projects-to-main.py
   git diff-files --quiet
+
+[testenv:normalize-projects-yaml]
+deps =
+  ruamel.yaml
+commands =
+  {toxinidir}/tools/normalize_projects_yaml.py