feat(router): support customized server_names_hash_bucket_size setting

Long domain names have trouble with the default settings for
server_names_hash_bucket_size and server_names_hash_max_size. This
commit makes these settings configurable based on etcd keys.

See http://nginx.org/en/docs/http/server_names.html for details
on these settings.
Merge pull request #2550 from carmstrong/router-configure_server_name_size

feat(router): support customized server_names_hash_bucket_size setting