Fix nginx DNS resolution of restarted services

Previously, a common error when restarting e.g: the lms or the cms was
that nginx redirected to the wrong container. For instance:

    access studio.localhost
    tutor local restart lms cms
    access studio.localhost

In the second call to studio.localhost, we were frequently trying to
access the LMS, which resulted in a 400 error.

We solve this issue by setting a TTL of 10s on the nginx proxy name
resolution.

More docs:
http://nginx.org/en/docs/http/ngx_http_core_module.html#resolver

CHANGELOG.md

@@ -4,6 +4,7 @@ Note: Breaking changes between versions are indicated by "💥".
 
 ## Unreleased
 
+- [Bugfix] Fix DNS resolution of restarted service
 - [Feature] Restart multiple services with `local restart`
 - [Feature] Make it possible to easily reload openedx gunicorn process with `tutor local exec lms reload-gunicorn``
 - [Improvement] Rename lms/cms_worker to lms/cms-worker in local deployment

tutor/templates/apps/nginx/tutor.conf

@@ -1,2 +1,5 @@
 # Allow long domain names
 server_names_hash_bucket_size 128;
+
+# Set a short ttl for proxies to allow restarts
+resolver 127.0.0.1 [::1]:5353 valid=10s;