config: Add a function to provision mutable service

config: Add a function to provision mutable service

The point here is to create a virtual configuration, which does not come
from a real file. This is a handy way for plugins to be able to provision
services without creating any file on the FS.

In case of a wifi configuration type and if connect is requested, it will
trigger a scan, thus leading to a possible service being provisioned by
such virtual configuration. If so and if connect was requested: the service
will be asked to connect.