[Brightbox] Servers can now have their groups updated

The support was already there in fog, this code just adds documentation
comment for it
[Brightbox] Switches to new format helper

Using #data_matches_schema which correctly fails when a nullable
resource is absent.
[Brightbox] Expands on some tests

Starts checking fields that should have been altered by a request have.
[Brightbox] Adds missing images attributes
[Brightbox] Breaks schemas from test helper
Merge pull request #2124 from brightbox/brightbox_updates

Brightbox updates