Fixes #92 (Config slice returns data inconsistent with other slices)

The changes in this pull request ensure that the RESTful API of the `config` slice returns a `slice_success` object in response to a **GET** against the `/config` endpoint and that the response body of a **GET** against the `/config/ipxe` endpoint (which contains the text of the iPXE boot script that should be used to configure the chain-boot of nodes into Hanlon from the TFTP server used for PXE-booting) is handled properly.

With these changes, we can now get rid of two methods from the `ProjectHanlon::HttpHelper` module (the `hnl_http_get_hash_response` and `hnl_http_get_text` methods) and handle all **GET** responses using a common codebase (with common error handling).  With these changes in place, the interface between the CLI and RESTful API for the config slice is much simpler, and error handling should be much more robust (since a common codebase is used between the CLI and RESTful API for **all** slices).