NSX: unify the two distributed routing extensions

This is done by adopting the router_extra_attributes
models. However, the NSX 'distributed' extension is
slightly different in that it is visible to tenants
and prevents router conversion (allow_put=false).

PUT requests are made return the correct HTTP code;
The access control misalignments will need to be
adjusted via rules in policy.json. This will be
properly documented.

DocImpact

Supports-blueprint: neutron-ovs-dvr

Change-Id: I35a4b54318d8169eb2c73be77ca2f30bbee08b46
Merge "NSX: unify the two distributed routing extensions"