Add DB mappings with NSX logical routers

This patch introduces DB mappings between neutron and NSX router,
thus not requiring anymore the Neutron router ID to be equal to the
NSX one.

This change is needed for enabling asynchronous operations in
the NSX plugin.

This patch also performs NVP/NSX renaming where appropriate, and
fixes delete router logic causing a 500 HTTP error to be returned
when a Neutron internal error occurs.

Related to blueprint nvp-async-backend-communication
Related to blueprint nicira-plugin-renaming

Change-Id: Ib0e9ed0f58e7fa3497a93e9cd3baa9cb81ad797b