Make vault load balancer sticky

Assets like css and js were not proxying correctly. I think it may be
because they were proxying to a different instance and that the paths
are dynamic. This should route subsequent requests for the session to
a single backend.

nomad/traefik/traefik.nomad

@@ -220,6 +220,7 @@ CF_ZONE_API_TOKEN={{ .Data.data.api_token_zone_read }}
     {{ with service "vault" -}}
     [http.services.vault]
       [http.services.vault.loadBalancer]
+        [http.services.vault.loadBalancer.sticky.cookie]
         {{ range . -}}
         [[http.services.vault.loadBalancer.servers]]
           url = "http://{{ .Address }}:{{ .Port }}"