Allow yaml configuration file overrides per-service

[tatu-at-datastax]

Currently all Stargate V1 services are based on DropWizard, and include src/main/resources/config.yaml for their default settings. While these settings can theoretically be overridden with System Properties (-Ddw.http.port=8088), there is no prefixing to avoid changing settings for all.
Since yaml configuration files are baked in jars, changing them is difficult as well.

To support things like configuring https endpoints for services it makes sense to allow specifying alternate configuration files, on per-service basis. This can be done by, for example:

1. Adding new System properties that, if specified, will point to file to read instead (./my-service-config/rest-config.yaml) of the default resource
2. Using naming convention (for example [SERVICE-NAME]-config.yaml, like restapi-config.yaml) and check for existence of such file, and if found using as replacement

It seems simple enough to do both, in this order of precedence.

[tatu-at-datastax]

Implementation of overrides ready, with tests.