Fatbuildrweb is unable to list available instances when instance default does not exist

[rezib]

Example:

$ curl -k https://build.dev.rackslab.io/instances.json
{"error":"instance default not found"}

Fatbuildrweb tries to connect to fatbuildrd on default instance to retrieve the list of defined instances. But it fails when default instance is not defined. Another way to retrieve defined instances must be found.

[rezib]

Fixed with 78c9449.