Don't override transport_url with old configs

The backwards compatibility support turned out being a forward
compatibility blocker. This patch attempts to first load the notifier
transport using the transport_url and then, if the transport_url was not
configured, it loads the transport by using the old strategy config
params.

Closes-bug: #1263945

Change-Id: I4c80c64e2d1afaf518886b51a50300a576ee6317
Merge "Don't override transport_url with old configs"