cubicweb #1528533 work with pyro but without pyrons [validation pending]

I need to run CW with a pyro server but no pyrons. To achieve this, the registering to pyrons can fail because no NS is found, but this must not prevent CW from starting.

The server must have a way of knowing its pyro URL so that it can be queried by cubes which will then be able to give that URL to e.g. scripts requiring it.