Cleanup set/get_env related code in ns_babysitter

Opening this ticket based on discussion with Aliaksey.

ns_babysitter has setup_static_config duplicated from ns_server.erl to set ns_sever env variables. It does that so that it can do application:get_env(ns_server, ...) later.

Instead, it will be good if it sets/gets the env from ns_babysitter app itself.

This ticket should:

1. Remove ns_server reference from setup_static_config - so that it does
get and set on the application of the calling process.
2. Remove duplicate setup_static_config() function. Same function should be called by both ns_server and ns_babysitter.
3. Remove ns_server references from ns_babysitter in various application:get_env(ns_server, ...) calls.
4. Any other cleanup as needed.