Process all relevant cluster resources at start (nginx#184)

Problem:
At start, the Gateway generates the first NGINX configuration (and
a few subsequent ones) before it processed all relevant cluster resources.
Such configuration is incomplete and should not be generated. For clients
it means they can see transient 404 errors when NGINX is configured with
such configuration.

Solution:
When the Gateway starts, it will generate the first NGINX configuration
based on processing all relevant cluster resources.

Author: miledxz