install: Check nginx configuration is valid.

It's better to fail here and have the user correct the issue than fail
later.

scripts/lib/install

@@ -21,6 +21,9 @@ echo -e "[machine]\npuppet_classes = zulip::voyager\ndeploy_type = voyager" > /e
 # These server restarting bits should be moveable into puppet-land, ideally
 apt-get -y upgrade
 if [ -e "/etc/init.d/nginx" ]; then
+    # Check nginx was configured properly now that we've installed it.
+    # Most common failure mode is certs not having been installed.
+    nginx -t
     service nginx restart
 fi