Change the satellite Admin HTTP server for:
* Embedding the UI assets into the Go binary.
* Serve the UI assets from the embedded file system or from a specific
directory path through a configuration flag, without requiring
authentication but keeping the authentication verification for the API
endpoints.
* Add tests to verify that the UI assets are served without
authentication.
Change-Id: I9003ac96f1ec585a189b67fc1cb315905403d557
Fix the Admin API endpoints when it response with a client error
response code.
The most of the client response error send a JSON body but the
`Content-Type` header wasn't set to the appropriated value. This commit
fixes them and it adds assertions to the tests to very the
`Content-Type` header.
Updates the README to briefly document about the format of the client
errors endpoints responses.
Change-Id: Ifaf2122def801701211438ce241046be1adc0e8c
