agama config load/store for "storage" uses the HTTP API

[mvidner]

Problem

• https://trello.com/c/hvPtBtMD

To recap:

When moving to the new HTTP-based architecture, we took some shortcuts. One of them was not using HTTP clients in the command-line interface.

Solution

• Add StorageHTTPClient
• This 🛑 ends D-Bus 🛑 usage from SettingsStore ✔️

Testing

• Added a new unit test
• Tested manually: note that it is easy to pass invalid Storage config in this way which will confuse the UI to the point of not rendering the storage parts at all.

Screenshots

No

[coveralls]

coverage: 72.447% (-0.007%) from 72.454%
when pulling dbeb055 on http-client-storage
into bc057f4 on master.

[mvidner]

Hmm, after load worked on the first try I wondered how store would work. And it does! ... by not validating anything? Is that right?

# echo '{"storage":{"bogus": "validate much?"}}' | agama config load
# agama config show
{
  "user": {
    "...": "...".
  },
  "storage": {
    "bogus": "validate much?"
  }
}

[imobachgs]

Congrats! I have added just a minor comment but, otherwise, it LGTM.