Return 400 if creating a distributed router on old NVP platforms

Without checking for the right NVP support, creating a distributed
router will silently fail, i.e. ithe 'distributed' attribute will
be ignored and a centralized router will be created instead.

Supports blueprint nvp-distributed-router

Change-Id: If8e40c0907d78c30f9ba918aaaff57e58ebb8055
Merge "Return 400 if creating a distributed router on old NVP platforms"