lttng-ctl: add override helper to load api

lttng-ctl: add override helper to load api

User can provide an url override that will be used during the loading of a
configuration file.

The url format is the same found at lttng-create(1).

This correspond to a --set-url operation on lttng-create.

Note: an url might be invalid for a trace configuration. The override will fail
during the session load (call to lttng_load_session) and simply cancel the
loading.

e.g Loading a live session with a file:// url override will fail.