read default bootstrap servers from file

Default bootstrap servers are now read from the following files:
 - share/<APPNAME>/bootstrap/servers.json
 - share/<APPNAME>/bootstrap/servers.json.d/*.json

Should those files be absent in all QStandardPaths::Datalocation directories,
empty, or contain invalid JSON, they will be ignored. In the event where no
bootstrap servers can be loaded from file, it will default to
bootstrap.ring.cx.

Tuleap: #611
Change-Id: I5e81b5338097c46a5d68135ed3dc38cd9912edf1