Tuning server names hash

[stuartpb]

See #2, plushu/plushu-app-nginx-servers#2

This could be done with hooks like vhost-domains and vhost-config-changed (and maybe another plugin that implements these hooks): the part of the Nginx config regarding server names can be set based on the needed server names (with a certain configurable buffer value extra - I'm thinking 8 by default), setting the number to the number of vhost domains, and the size to the longest vhost domain.