Details on NVP implementation of Provider network extension

Provide more information on how flat and vlan networks are supported in
the backend, and discuss how the max_lp_per_bridged_ls should be tuned
according to the NVP version running in the backend

Bug 1227592

Change-Id: I339c4f0eb9f78d586d249d754084962387db8579
Merge "Details on NVP implementation of Provider network extension"