make default transport type configurable nvp

Fixes bug 1179759

Change-Id: I8f644f2bc0d5d144f52e5ed0bd2e991503581ad2
Add missing unit test for NVP metadata_mode option

Note: I didn't squash this change in my patch that made the default
transport type configurable because this change was not in grizzly
and we do not want to backport it.

Change-Id: Ie71e5e56d1bed219caba74799cc2310ace987b8f