Handle NetworkAmbiguous error when booting a new instance with v3 api

Catch exception.NetworkAmbiguous error and map it to exc.HttpConflict when
using v3 api to create a new instance.

Change-Id: I18b6b9f0d8612a3757f6489dada41b5cd8eca758
Closes-Bug: 1350800
Merge "Handle NetworkAmbiguous error when booting a new instance with v3 api"