I gradually moved one key after the other out of the keys file because it made the code much easier to understand. 

Things that are constants and never change should not be in a properties file.  e.g. HTTP_POST_METHOD = POST  This is never going to change. The POST verb of REST will always be called POST. It just adds complexity when you put it into a config/properties file. 

That's true with most of the items that were in this file.

Things that can change like endpoints on specific REST API providers could/should be in their corresponding properties files. 
As should be things like username/pwd that will change between each user.

Almost all the items in the config file were used in only one place, so much easier to understand the code when they are actually defined in this place and you don't have to do the indirection.