In favor of the private i18n wrapper (_i18n), the public 'i18n' wrapper
has been deprecated for several releases. Each neutron CLI plugin
(not OSC plugin) should use its own i18n wrapper and the usage of
neutronclient.i18n wrapper was deprecated. It is time to drop this.
Also fixes test_exception.py which still consumed neutronclient.i18n.
Change-Id: I27a8ace2f31bea5e1d6bf8a5ebbbfa52a3228e73