translator: remove dependency of neutron.spi

translator: remove dependency of neutron.spi

netvirt still had a dependency on netutron.spi. which seems accidentally
remained when netvirt migrated from I*Aware AD-SAL to MD-SAL.
This patch removes its dependency and finish its migration with the only
dependency is neutron yang model.
In order to avoid circular dependency, NeutronModelsDataStoreHelper was
moved from neutron.util to net-virt.

Change-Id: I09628c39a2df85bc18d930639965d5a466134658
Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>