Enable core API-v3 to be optional when unit testing

Long ago init_only was added to avoid load all extension during unit tests and
speed it up.
So this patch enables to *not* load all core API-v3.

Related to bp nova-v3-api

Change-Id: I6acd264ace4440724b944009dff8575df8ad3512
Merge "Enable core API-v3 to be optional when unit testing"