blockerbugs: changing SQLALCHEMY option in stg to reduce cron email

There was a change with a recent upgrade to flask-sqlalchemy so that
SQLALCHEMY_TRACK_MODIFICATIONS will no longer be enabled by default
in future releases. As we're not using it, I'm explicitly setting that
to False in the hopes that most/some of the cron emails go away if
the warning isn't emitted during blockerbugs sync.

roles/blockerbugs/templates/blockerbugs-settings.py.j2

@@ -7,6 +7,7 @@ FAS_PASSWORD = "{{ blockerbugs_fas_password }}"
 FAS_FLASK_COOKIE_REQUIRES_HTTPS = False
 FAS_CHECK_CERT = False
 PRODUCTION = False
+SQLALCHEMY_TRACK_MODIFICATIONS = False
 {% endif %}
 BUGZILLA_URL = '{{ blockerbugs_bugzilla_url }}'
 BODHI_URL = '{{ blockerbugs_bodhi_url }}'